U.S. patent number 10,861,121 [Application Number 15/740,172] was granted by the patent office on 2020-12-08 for methods of manufacturing security documents and security devices.
This patent grant is currently assigned to DE LA RUE INTERNATIONAL LIMITED. The grantee listed for this patent is DE LA RUE INTERNATIONAL LIMITED. Invention is credited to Lawrence George Commander, Ian Cornes, John Godfrey, Brian William Holmes, John O'Malley.
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United States Patent |
10,861,121 |
Holmes , et al. |
December 8, 2020 |
Methods of manufacturing security documents and security
devices
Abstract
A method of manufacturing a security document, including:
providing a polymer substrate having first and second surfaces;
and: applying an array of focussing elements to the first surface
of the polymer substrate across a first region; forming an image
array by: providing a die form having a surface including an
arrangement of raised areas and recessed areas defining the
pattern; applying a first curable material to the surface of the
die form so it substantially fills the recessed areas; bringing a
pattern support layer in contact with the surface of the die form
so it covers the recessed areas; separating the pattern support
layer from the surface of the die form so the first curable
material in the recessed areas is removed from the recessed areas
and retained on the pattern support layer; and at least partly
curing the first curable material in one or more curing steps.
Inventors: |
Holmes; Brian William (Fleet,
GB), Godfrey; John (London, GB), Cornes;
Ian (Bolton, GB), O'Malley; John (Manchester,
GB), Commander; Lawrence George (Reading,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
DE LA RUE INTERNATIONAL LIMITED |
Baskingstoke |
N/A |
GB |
|
|
Assignee: |
DE LA RUE INTERNATIONAL LIMITED
(Hampshire, GB)
|
Family
ID: |
1000005231647 |
Appl.
No.: |
15/740,172 |
Filed: |
July 11, 2016 |
PCT
Filed: |
July 11, 2016 |
PCT No.: |
PCT/GB2016/052081 |
371(c)(1),(2),(4) Date: |
December 27, 2017 |
PCT
Pub. No.: |
WO2017/009616 |
PCT
Pub. Date: |
January 19, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180186166 A1 |
Jul 5, 2018 |
|
Foreign Application Priority Data
|
|
|
|
|
Jul 10, 2015 [GB] |
|
|
1512118.9 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B42D
25/351 (20141001); G02B 3/005 (20130101); G06K
9/3216 (20130101); G06T 7/80 (20170101); B29D
11/00365 (20130101); G02B 3/0031 (20130101); G06T
1/0014 (20130101); B42D 25/355 (20141001); B42D
25/45 (20141001); H04N 1/00267 (20130101); H04N
5/23212 (20130101); G02B 1/041 (20130101); B42D
25/342 (20141001); B42D 25/425 (20141001); B29D
11/00442 (20130101); B42D 25/455 (20141001); B42D
25/378 (20141001); G06K 9/3275 (20130101); B29D
11/00298 (20130101); B42D 25/324 (20141001); B41M
3/14 (20130101); G02B 3/0012 (20130101); B42D
25/48 (20141001); B42D 25/46 (20141001); H04N
5/247 (20130101); G02B 3/0006 (20130101); B42D
25/47 (20141001); B42D 25/24 (20141001); B42D
25/23 (20141001); G06T 2207/30144 (20130101); B42D
25/29 (20141001); G06K 2009/3225 (20130101) |
Current International
Class: |
G06T
1/00 (20060101); B41M 3/14 (20060101); G02B
1/04 (20060101); G06T 7/80 (20170101); G06K
9/32 (20060101); H04N 1/00 (20060101); H04N
5/232 (20060101); H04N 5/247 (20060101); B42D
25/425 (20140101); G02B 3/00 (20060101); B29D
11/00 (20060101); B42D 25/351 (20140101); B42D
25/342 (20140101); B42D 25/48 (20140101); B42D
25/378 (20140101); B42D 25/355 (20140101); B42D
25/46 (20140101); B42D 25/324 (20140101); B42D
25/45 (20140101); B42D 25/455 (20140101); B42D
25/24 (20140101); B42D 25/23 (20140101); B42D
25/29 (20140101); B42D 25/47 (20140101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1 008 616 |
|
Jun 2000 |
|
EP |
|
2490780 |
|
Nov 2012 |
|
GB |
|
2503783 |
|
Jan 2014 |
|
GB |
|
2520605 |
|
May 2015 |
|
GB |
|
94/27254 |
|
Nov 1994 |
|
WO |
|
2005/052650 |
|
Jun 2005 |
|
WO |
|
2005/106601 |
|
Nov 2005 |
|
WO |
|
2008/042631 |
|
Apr 2008 |
|
WO |
|
2011/051668 |
|
May 2011 |
|
WO |
|
2011/051669 |
|
May 2011 |
|
WO |
|
2011/051670 |
|
May 2011 |
|
WO |
|
2011/102800 |
|
Aug 2011 |
|
WO |
|
2011/107782 |
|
Sep 2011 |
|
WO |
|
2011/107783 |
|
Sep 2011 |
|
WO |
|
2011/116425 |
|
Sep 2011 |
|
WO |
|
2012/027779 |
|
Mar 2012 |
|
WO |
|
2013/167887 |
|
Nov 2013 |
|
WO |
|
2014/000020 |
|
Jan 2014 |
|
WO |
|
2014/070079 |
|
May 2014 |
|
WO |
|
2014/184559 |
|
Nov 2014 |
|
WO |
|
2015/011493 |
|
Jan 2015 |
|
WO |
|
2015/011494 |
|
Jan 2015 |
|
WO |
|
2015/044671 |
|
Apr 2015 |
|
WO |
|
Other References
Jan. 8, 2016 Search Report issued in United Kingdom Patent
Application No. GB 1512118.9. cited by applicant .
Oct. 31, 2017 Combined Search Report and Examination Report issued
in United Kingdom Patent Application No. GB 1709454.1. cited by
applicant .
Sep. 26, 2017 Combined Search Report and Examination issued in
United Kingdom Patent Application No. GB 1714332.2. cited by
applicant .
Dec. 8, 2016 International Search Report and Written Opinion issued
in International Patent Application No. PCT/GB2016/052081. cited by
applicant .
Jan. 16, 2018 International Preliminary Report on Patentability
issued in International Patent Application No. PCT/GB2016/052081.
cited by applicant.
|
Primary Examiner: Banh; David H
Attorney, Agent or Firm: Oliff PLC
Claims
The invention claimed is:
1. A method of manufacturing a security device, comprising:
providing a transparent support layer having first and second
surfaces; conveying the transparent support layer along a transport
path in a machine direction; and during the conveying,
simultaneously at the same position along the transport path in the
machine direction: (a) forming an array of focussing elements on
the first surface of the transparent support layer in at least a
first region by: (a)(i) applying at least one transparent curable
material either to the transparent support layer or to a casting
tool, carrying a surface relief corresponding to the focussing
elements, over an area which includes at least the first region;
(a)(ii) forming the at least one transparent curable material with
the casting tool; and (a)(iii) curing the at least one transparent
curable material so as to retain the surface relief in the first
region; and (b) applying an image array to the second surface of
the transparent support layer in at least part of the first region
by printing the image array onto the transparent support layer via
one of: intaglio printing, gravure printing, wet lithographic
printing, dry lithographic printing, or flexographic printing;
wherein the array of focussing elements and the image array are
registered to one another at least in the machine direction.
2. The method according to claim 1, wherein: in step (a), a
focussing element cylinder carrying the surface relief on its
circumference corresponding to the array of focussing elements is
used as the casting tool to form the array of focussing elements on
the first surface of the transparent support layer, in step (b), an
image cylinder is used to apply the image array to the second
surface of the transparent support layer, steps (a) and (b) are
performed simultaneously at a nip formed between the focussing
element cylinder and the image cylinder, and the transparent
support layer passes through the nip.
3. The method according to claim 2, wherein: the transport path is
configured such that the transparent support layer is held in
contact with the focussing element cylinder over a portion of its
circumference between a first contact point and a last contact
point spaced from one another by a non-zero distance, and the nip
formed between the focussing element cylinder and the image
cylinder either is located between the first and last contact
points, closer along the transport path to the last contact point
than to the first contact point, or forms the last contact
point.
4. The method according to claim 1, wherein: a focussing element
cylinder constitutes the casting tool, and step (a)(iii) is
performed while the transparent support layer is held in contact
with the focussing element cylinder over the portion of its
circumference such that the at least one transparent curable
material is at least partly cured at a location of a nip formed
between the focussing element cylinder and an image cylinder used
to apply the image array to the second surface of the transparent
support layer.
5. A method of manufacturing a security document, comprising:
providing a polymer substrate having first and second surfaces in
the form of a web; applying at least one opacifying layer to the
first and/or second surfaces of the polymer substrate in the form
of a web, the or each opacifying layer comprising a non-transparent
material; and then cutting the web into sheets in the direction of
web transit, then performing on the sheets, in at least one
sheet-fed process: (a) applying an array of focussing elements to
the first surface of the polymer substrate across a first region
by: (a)(i) applying at least one transparent material either to the
polymer substrate or to a casting tool carrying a surface relief
corresponding to the focussing elements, over an area that includes
at least the first region; (a)(ii) forming the at least one
transparent curable material with the casting tool; and (a)(iii)
curing the at least one transparent curable material so as to
retain the surface relief in the first region; and (b) applying an
image array to the polymer substrate in the first region, such that
the image array is located in a plane spaced from the array of
focussing elements by a distance substantially equal to the focal
length of the focussing elements via one of: intaglio printing,
gravure printing, wet lithographic printing, dry lithographic
printing, or flexographic printing, wherein: the focussing elements
exhibit a substantially focussed image of the image array; and
either the image array is located between the array of focussing
elements and the at least one opacifying layer on the first surface
of the substrate, or at least the opacifying layer(s) on the first
surface of the substrate define a gap forming a window region in
which at least part of the array of focussing elements is disposed
such that a substantially focussed image of at least part of the
image array is displayed in the window region.
6. The method according to claim 5, further comprising, after
cutting the web into sheets: printing a graphics layer onto the at
least one opacifying layer on the first and/or second surfaces of
the polymer substrate in at least one sheet-fed process.
7. The method according to claim 5, wherein: in step (b) the image
array is provided on the first surface of the polymer substrate,
and the focussing element array includes an optical spacing
portion.
8. A method of manufacturing a security document, comprising:
providing a polymer substrate having first and second surfaces in
the form of a web; and in either order: (i) applying at least one
opacifying layer to the first and/or second surfaces of the polymer
substrate in the form of a web, the or each opacifying layer
comprising a non-transparent material; and (ii) applying an array
of focussing elements to the first surface of the polymer substrate
in the form of a web across a first region by: applying at least
one transparent curable material either to the polymer substrate or
to a casting tool carrying a surface relief corresponding to the
focussing elements, over an area that includes at least the first
region; forming the at least one transparent curable material with
the casting tool; and curing the at least one transparent curable
material so as to retain the surface relief in the first region;
and then after steps (i) and (ii): cutting the web into sheets in
the direction of web transit; then performing on the sheets, in at
least one sheet-fed process: applying an image array to the polymer
substrate in the first region, such that the image array is located
in a plane spaced from the array of focussing elements by a
distance substantially equal to the focal length of the focussing
elements via one of: intaglio printing, gravure printing, wet
lithographic printing, dry lithographic printing, or flexographic
printing; wherein: the focussing elements exhibit a substantially
focussed image of the image array; and either the image array is
located between the array of focussing elements and the at least
one opacifying layer on the first surface of the substrate, or at
least the opacifying layer(s) on the first surface of the substrate
define a gap forming a window region in which at least part of the
array of focussing elements is disposed such that a substantially
focussed image of at least part of the image array is displayed in
the window region.
9. The method according to claim 8, further comprising, after
cutting the web into sheets: printing a graphics layer onto the at
least one opacifying layer on the first and/or second surfaces of
the polymer substrate in at least one sheet-fed process.
10. The method according to claim 8, wherein: the image array is
provided on the first surface of the polymer substrate, and the
focussing element array includes an optical spacing portion.
11. A method of manufacturing a security document, comprising:
providing a polymer substrate having first and second surfaces in
the form of a web; and in either order: (i) applying at least one
opacifying layer to the first and/or second surfaces of the polymer
substrate in the form of a web, the or each opacifying layer
comprising a non-transparent material; and (ii) applying an image
array to the polymer substrate in the form of a web in a first
region via one of: intaglio printing, gravure printing, wet
lithographic printing, dry lithographic printing, or flexographic
printing; and then after steps (i) and (ii): cutting the web into
sheets in the direction of web transit; then performing on the
sheets, in at least one sheet-fed process: applying an array of
focussing elements to the first surface of the polymer substrate
across the first region, such that the image array is located in a
plane spaced from the array of focussing elements by a distance
substantially equal to the focal length of the focussing elements
by: applying at least one transparent curable material either to
the polymer substrate or to a casting tool carrying a surface
relief corresponding to the focussing elements, over an area that
includes at least the first region; forming the at least one
transparent curable material with the casting tool; and curing the
at least one transparent curable material so as to retain the
surface relief in the first region; wherein: the focussing elements
exhibit a substantially focussed image of the image array; and
either the image array is located between the array of focussing
elements and the at least one opacifying layer on the first surface
of the substrate, or at least the opacifying layer(s) on the first
surface of the substrate define a gap forming a window region in
which at least part of the array of focussing elements is disposed
such that a substantially focussed image of at least part of the
image array is displayed in the window region.
12. The method according to claim 11, further comprising, after
cutting the web into sheets: printing a graphics layer onto the at
least one opacifying layer on the first and/or second surfaces of
the polymer substrate in at least one sheet-fed process.
13. The method according to claim 11, wherein: the image array is
provided on the first surface of the polymer substrate, and the
focussing element array includes an optical spacing portion.
14. The method according to claim 1, wherein the casting tool is a
focussing element cylinder.
15. The method according to claim 6, wherein the graphics layer is
printed onto the at least one opacifying layer before performing
step (a) and/or step (b).
16. The method according to claim 9, wherein the graphics layer is
printed onto the at least one opacifying layer before applying the
image array to the polymer substrate.
17. The method according to claim 12, wherein the graphics layer is
printed onto the at least one opacifying layer before applying the
array of focussing elements to the first surface of the polymer
substrate.
Description
This invention relates to methods of manufacturing security
documents and security devices, and to the corresponding products.
Security devices are typically used on security documents such as
banknotes, cheques, passports, identity cards, certificates of
authenticity, fiscal stamps and other secure documents, in order to
confirm their authenticity.
Articles of value, and particularly documents of value such as
banknotes, cheques, passports, identification documents,
certificates and licences, are frequently the target of
counterfeiters and persons wishing to make fraudulent copies
thereof and/or changes to any data contained therein. Typically
such objects are provided with a number of visible security devices
for checking the authenticity of the object. By "security device"
we mean a feature which it is not possible to reproduce accurately
by taking a visible light copy, e.g. through the use of standardly
available photocopying or scanning equipment. Examples include
features based on one or more patterns such as microtext, fine line
patterns, latent images, venetian blind devices, lenticular
devices, moire interference devices and moire magnification
devices, each of which generates a secure visual effect. Other
known security devices include holograms, watermarks, embossings,
perforations and the use of colour-shifting or
luminescent/fluorescent inks. Common to all such devices is that
the visual effect exhibited by the device is extremely difficult,
or impossible, to copy using available reproduction techniques such
as photocopying. Security devices exhibiting non-visible effects
such as magnetic materials may also be employed.
One class of security devices are those which produce an optically
variable effect, meaning that the appearance of the device is
different at different angles of view. Such devices are
particularly effective since direct copies (e.g. photocopies) will
not produce the optically variable effect and hence can be readily
distinguished from genuine devices. Optically variable effects can
be generated based on various different mechanisms, including
holograms and other diffractive devices, moire interference and
other mechanisms relying on parallax such as venetian blind
devices, and also devices which make use of focussing elements such
as lenses, including moire magnifier devices, integral imaging
devices and so-called lenticular devices.
Security devices comprising focussing elements typically require
the use of at least one transparent material either to act as an
optical spacer between the focussing elements and an image, or
image array, on which the focussing elements are to focus, or to
act as a support for the focussing element so that some other
object can be viewed therethrough. As such, security devices
comprising focussing elements are particularly well suited to
deployment on security documents based on polymer document
substrates, such as polymer banknotes, since the polymer document
substrate can be selected to be transparent and so provide one or
both of the above functions if desired. Therefore, in the main part
the present disclosure relates to polymer-based security
documents.
However, other aspects of the invention disclosed herein are not so
limited as will be made clear below. For example, the security
devices can be formed using a transparent material which is applied
to a security document of any sort, such as a conventional
paper-based document, e.g. in the form of a security article such
as a thread, strip, patch, foil or inserted which is incorporated
into or applied onto the security document.
Several aspects of the invention involve the provision of a
focussing element array and an image array located approximately in
the focal plane of the focussing element array such that the
focussing element array exhibits a substantially focussed image of
the image array. This focussed image may preferably be optically
variable and could for example be based on any of the mechanisms
detailed below. It should be appreciated that in all aspects of the
invention the focussing element array and image array could
optionally be configured to provide any one or more of these
effects, unless otherwise specified:
Moire magnifier devices (examples of which are described in
EP-A-1695121, WO-A-94/27254, WO-A-2011/107782 and WO2011/107783)
make use of an array of focusing elements (such as lenses or
mirrors) and a corresponding array of microimages, wherein the
pitches of the focusing elements and the array of microimages
and/or their relative locations are mismatched with the array of
focusing elements such that a magnified version of the microimages
is generated due to the moire effect. Each microimage is a
complete, miniature version of the image which is ultimately
observed, and the array of focusing elements acts to select and
magnify a small portion of each underlying microimage, which
portions are combined by the human eye such that the whole,
magnified image is visualised. This mechanism is sometimes referred
to as "synthetic magnification". The magnified array appears to
move relative to the device upon tilting and can be configured to
appear above or below the surface of the device itself. The degree
of magnification depends, inter alia, on the degree of pitch
mismatch and/or angular mismatch between the focusing element array
and the microimage array.
Integral imaging devices are similar to moire magnifier devices in
that an array of microimages is provided under a corresponding
array of lenses, each microimage being a miniature version of the
image to be displayed. However here there is no mismatch between
the lenses and the microimages. Instead a visual effect is created
by arranging for each microimage to be a view of the same object
but from a different viewpoint. When the device is tilted,
different ones of the images are magnified by the lenses such that
the impression of a three-dimensional image is given.
"Hybrid" devices also exist which combine features of moire
magnification devices with those of integral imaging devices. In a
"pure" moire magnification device, the microimages forming the
array will generally be identical to one another. Likewise in a
"pure" integral imaging device there will be no mismatch between
the arrays, as described above. A "hybrid" moire
magnification/integral imaging device utilises an array of
microimages which differ slightly from one another, showing
different views of an object, as in an integral imaging device.
However, as in a moire magnification device there is a mismatch
between the focusing element array and the microimage array,
resulting in a synthetically magnified version of the microimage
array, due to the moire effect, the magnified microimages having a
three-dimensional appearance. Since the visual effect is a result
of the moire effect, such hybrid devices are considered a subset of
moire magnification devices for the purposes of the present
disclosure. In general, therefore, the microimages provided in a
moire magnification device should be substantially identical in the
sense that they are either exactly the same as one another (pure
moire magnifiers) or show the same object/scene but from different
viewpoints (hybrid devices).
Moire magnifiers, integral imaging devices and hybrid devices can
all be configured to operate in just one dimension (e.g. utilising
cylindrical lenses) or in two dimensions (e.g. comprising a 2D
array of spherical or aspherical lenses).
Lenticular devices on the other hand do not rely upon
magnification, synthetic or otherwise. An array of focusing
elements, typically cylindrical lenses, overlies a corresponding
array of image sections, or "slices", each of which depicts only a
portion of an image which is to be displayed. Image slices from two
or more different images are interleaved and, when viewed through
the focusing elements, at each viewing angle, only selected image
slices will be directed towards the viewer. In this way, different
composite images can be viewed at different angles. However it
should be appreciated that no magnification typically takes place
and the resulting image which is observed will be of substantially
the same size as that to which the underlying image slices are
formed. Some examples of lenticular devices are described in U.S.
Pat. No. 4,892,336, WO-A-2011/051669, WO-A-2011051670,
WO-A-2012/027779 and U.S. Pat. No. 6,856,462. More recently,
two-dimensional lenticular devices have also been developed and
examples of these are disclosed in British patent application
numbers 1313362.4 and 1313363.2. Lenticular devices have the
advantage that different images can be displayed at different
viewing angles, giving rise to the possibility of animation and
other striking visual effects which are not possible using the
moire magnifier or integral imaging techniques.
Arrays of lenses or other focussing elements can also be used as a
security device on their own (i.e. without a corresponding image
array), since they can be used to exhibit a magnified or distorted
view of any background they may be placed against, or scene viewed
therethrough. This effect cannot be replicated by photocopying or
similar.
Aspects of the present invention provide improved methods of
manufacturing security documents comprising security devices of the
sorts described above.
A first aspect of the present invention provides a method of
manufacturing a security document, comprising:
providing a polymer substrate having first and second surfaces;
and, in any order:
(a) applying an array of focussing elements to the first surface of
the polymer substrate across a first region (b) forming an image
array comprising a pattern of a first curable material (preferably
having a visible colour), by: (b)(i) providing a die form, the die
form having a surface comprising an arrangement of raised areas and
recessed areas defining the pattern; (b)(ii) applying a first
curable material to the surface of the die form such that said
first curable material substantially fills the recessed areas;
(b)(iii) bringing a pattern support layer in contact with the
surface of the die form such that it covers the recessed areas;
(b)(iv) separating the pattern support layer from the surface of
the die form such that the first curable material in the recessed
areas is removed from said recessed areas and retained on the
pattern support layer in accordance with the pattern; and (b)(v)
during and/or after step (b)(ii), at least partly curing the first
curable material in one or more curing steps; wherein either the
pattern support layer comprises the polymer substrate or step (b)
further comprises applying the pattern support layer to the polymer
substrate, such that the image array is located in a plane spaced
from the array of focussing elements by a distance substantially
equal to the focal length of the focussing elements whereby the
focussing elements exhibit a substantially focussed image of the
image array; and (c) applying at least one opacifying layer to the
first and/or second surfaces of the polymer substrate, the or each
opacifying layer comprising a non-transparent material, wherein
either the image array is located between the array of focussing
elements and the at least one opacifying layer on the first surface
of the substrate, or at least the opacifying layer(s) on the first
surface of the substrate define a gap forming a window region in
which at least part of the array of focussing elements is disposed
such that a substantially focussed image of at least part of the
image array is displayed in the window region.
Security devices such as moire magnifiers, integral imaging devices
and lenticular devices, as well as others involving the use of
focusing elements, depend for their success significantly on the
resolution with which the image array (comprising e.g. microimages
or image elements) can be formed. Since the security device must be
thin in order to be incorporated into a document such as a
banknote, any focusing elements required must also be thin, which
by their nature also limits their lateral dimensions. For example,
lenses used in such security elements preferably have a width or
diameter of 50 microns or less, e.g. 30 microns. In a lenticular
device this leads to the requirement that each image element must
have a width which is at most half the lens width. For example, in
a "two channel" lenticular switch device which displays only two
images (one across a first range of viewing angles and the other
across the remaining viewing angles), where the lenses are of 30
micron width, each image element must have a width of 15 microns or
less. More complicated lenticular effects such as animation, motion
or 3D effects usually require more than two interlaced images and
hence each element needs to be even finer in order to fit all of
the image elements into the optical footprint of each lens. For
instance, in a "six channel" device with six interlaced images,
where the lenses are of 30 micron width, each image element must
have a width of 5 microns or less.
Similarly high-resolution image elements are also required in moire
magnifiers and integral imaging devices since approximately one
microimage must be provided for each focusing element and again
this means in effect that each microimage must be formed within a
small area of e.g. 30 by 30 microns. In order for the microimage to
carry any detail, fine linewidths of 5 microns or less are
therefore highly desirable.
Conventional processes used to manufacture image elements for
security devices are based on printing and include intaglio,
gravure, wet lithographic printing as well as dry lithographic
printing. The achievable resolution is limited by several factors,
including the viscosity, wettability and chemistry of the ink, as
well as the surface energy, unevenness and wicking ability of the
substrate, all of which lead to ink spreading. With careful design
and implementation, such techniques can be used to print pattern
elements with a line width of between 25 .mu.m and 50 .mu.m. For
example, with gravure or wet lithographic printing it is possible
to achieve line widths down to about 15 .mu.m.
In contrast, the method defined in step (b) above can be used to
achieve a very high resolution pattern, e.g. with pattern elements
of 10 microns line width or less. Exemplary implementations of the
step (b) method are described in WO-A-2014/070079,
US-A-2009/0297805 and WO-A-2011/102800, each of which is
incorporated herein by reference.
The present inventors have found that particular benefits are
achieved where this method of forming the image array is
incorporated into the production of a polymer-based security
document, such as a polymer banknote. One advantage is that all of
the recited components of the security document can, if desired, be
formed using web-based techniques meaning that all steps can
optionally be carried out as part of an in-line process, resulting
in fast, high-volume output of security documents (or at least a
security document precursor, ready for final printing and cutting).
Another advantage is that the polymer substrate of the document
itself can be transparent and act as an optical spacer between the
array of focussing elements and the image array. Since a polymer
document substrate can have a greater thickness than would be
permissible for an article such as a thread or strip which is to be
affixed to such a document substrate (e.g. approximately 70 microns
rather than approximately 30 microns), this increase in available
optical spacing lessens the constraints on the size of the elements
forming the image array. Nonetheless by utilizing the high
resolution image array formation technique defined in step (b) not
only can the remaining constraints be met but they can be met
comfortably, allowing for the creation of more complex effects,
e.g. a greater number of channels in a lenticular device.
Thus, preferably the image array is located on the second surface
of the polymer substrate. However, in other cases the image array
or a second image array could be provided on the first surface of
the polymer substrate, e.g. if the focussing element array is
formed in an additional transparent layer applied to the first
surface of the polymer substrate and itself provides the necessary
optical spacing. Such an additional transparent layer could be
provided by laminating a component carrying the focussing element
array onto the first surface, or by cast curing for instance.
In a particularly preferred embodiment, step (b)(ii) comprises
applying the first curable material to the surface of the die form
in at least two sequential application steps such that any of the
recessed areas not substantially filled in the first application
step are substantially filled in the second or subsequent
application step(s). In this case, the curing could all take place
once all the application steps have been completed. However,
preferably step (b)(v) comprises at least partially curing the
first curable material in the recessed areas between each
sequential application step in step (b)(ii). In a particularly
preferred implementation, the first curable material applied to the
surface of the die form in the last of the at least two sequential
application steps is only partially cured (if at all) before step
(b)(iii) and fully cured once the pattern support layer has been
brought in contact with the die form. Thus, the preceding
application(s) of curable material may be fully cured before the
last is applied which remains at least partly uncured when it
contacts the pattern support layer, thereby improving adhesion with
the support layer. The material can then be fully cured once in
contact resulting in a particularly strong bond.
In step (b)(ii) the first curable material could be applied in such
a way that it is only deposited into the recessed areas, e.g. by
appropriate selection of its viscosity or by application using a
doctor chamber. However, preferably step (b)(ii) further comprises
removing any excess first curable material from the surface of the
die form outside the recessed areas, preferably using a doctor
blade or by polishing.
In a particularly preferred embodiment, step (b) further comprises,
after step (b)(ii) and before step (b)(iii): (b)(ii') covering the
surface of the die form and the recessed areas filled with the
first curable material with a second curable material; wherein step
(b)(v) further comprises at least partly curing the second curable
compound, and in step (b)(iii) the pattern support layer contacts
the second curable material on the surface of the die form such
that in step (b)(iv) the second curable material is additionally
retained on the pattern support layer, the first curable material
being retained on the second curable material in accordance with
the pattern. This technique has been found to improve the bonding
between the pattern elements and the pattern support layer.
Alternatively, step (b) may advantageously further comprise, before
step (b)(iii): (b)(ii'') applying a second curable material to the
pattern support layer; wherein step (b)(v) further comprises at
least partly curing the second curable compound, and in step
(b)(iii) the second curable material on the surface of the pattern
support layer contacts the die form such that in step (b)(iv) the
first curable material is retained on the second curable material
in accordance with the pattern. In this case it is desirable that
the pattern support layer carrying the second curable material is
pressed against the surface of the die form with some pressure to
ensure good bonding.
In some embodiments, the second curable material will be
transparent so that the pattern can be viewed from either side
(provided the pattern support is also transparent). However in some
preferred implementations, the second curable material is a
substantially non-transparent material in the visible spectrum, the
non-transparent second curable material preferably forming one of
the at least one opacifying layers applied in step (c). Thus, the
second curable material may extend over substantially the whole of
the polymer substrate except for any desired window regions. The
said opacifying layer can be used as an optical barrier in a
dual-sided device as described in a later aspect of the
invention.
In further preferred embodiments, the second curable material is
configured to act as an anti-static layer. Advantageously,
therefore, the second curable material comprises an electrically
conductive substance. For instance, the second curable material
could contain a dispersion of conductive particles such as
graphite, or could comprise an additive such as any of those
disclosed in EP1008616, WO2014/000020 and WO2008/042631.
Transparent conductive additives are particularly preferred so that
the appearance of the layer is substantially unaffected. However as
mentioned above the second curable material as a whole may be
either transparent or non-transparent.
The second curable material preferably has enhanced adhesion
properties relative to the first curable material such that it acts
to help retain the first curable material on the support layer.
In a particularly preferred implementation, the first curable
material retained on the pattern support layer in step (b)(iv) is
in the form of a plurality of printed features which correspond to
the pattern, each of the plurality of printed features projecting
away from the pattern support layer to form a raised surface of the
plurality of printed features, the plurality of printed features
being separated from each other by gaps in the first curable
material on the pattern support layer, and step (b) further
comprises, after step (b)(iv) and preferably step (b)(v): (b)(vi)
applying at least one optically detectable material to at least a
portion of the pattern support layer either such that the at least
one optically detectable material is present on only the raised
surface of the printed features, and is substantially not present
in the gaps in the first curable material on the pattern support
layer, which separate the printed features, or such that the at
least one optically detectable material is present on only in the
gaps in the first curable material on the pattern support layer,
which separate the printed features and is substantially not
present on the raised surface of the printed features.
This approach allows the appearance of the pattern to be modified
by the application of the at least one optically detectable
material. Thus, the first curable material could be of any
appearance (including transparent and/or colourless) whilst the
optically detectable material can for instance have a colour which
contrasts with the underlying support layer in order to render the
pattern visible, or machine readable if the optically detecting
material emits/reflects outside the visible spectrum only (e.g. UV
luminescence). The at least one optically detectable material can
be placed only in the recesses or only on the raised surfaces by
controlling the parameters of the application process in step
(b)(vi), e.g. the viscosity of the material(s) and the pressure and
temperature at which they are applied. Exemplary processes and
suitable parameters for achieving this are disclosed in
US20110045248. Different materials could also be applied into the
recesses and onto the tops of the relief, respectively.
Advantageously, at least two optically detectable materials with
different optical detection characteristics may be applied to
laterally-offset regions of the pattern in step (b)(vi). For
example, the at least two materials may have different visible
colours. In this way a multi-coloured pattern or image can be
applied to the pattern and retained only in the recesses or only on
the tops of the relief, whereby the high-resolution nature of the
pattern is retained. The at least two materials are preferably
applied in register with one another, but do not need to be
registered to the pattern.
Preferably, the image array is configured so as to co-operate with
the array of focussing elements to generate an optically variable
effect. For example, in a particularly preferred embodiment, the
image array comprises a microimage array, and the pitches of the
focusing element array and of the microimage array and their
relative orientations are such that the focusing element array
co-operates with the microimage array to generate a magnified
version of the microimage array due to the moire effect. (Moire
magnifier)
In another case, the image array comprises a microimage array, the
microimages all depicting the same object from a different
viewpoint, and the pitches and orientation of the focusing element
array and of the microimage array are the same, such that the
focusing element array co-operates with the microimage array to
generate a magnified, optically-variable version of the object.
(Integral imaging device)
In a still further example, the image array comprises a set of
first image elements comprising portions of a first image,
interleaved with a set of second image elements comprising portions
of a second image, the focusing element array being configured such
that each focusing element can direct light from a respective one
of the first image elements or from a respective one of the second
image elements therebetween in dependence on the viewing angle,
whereby depending on the viewing angle the array of focusing
elements directs light from either the set of first image elements
or from the second image elements therebetween, such that as the
device is tilted, the first image is displayed to the viewer at a
first range of viewing angles and the second image is displayed to
the viewer at a second, different range of viewing angles.
(Lenticular device)
The focussing element array can be formed using various different
techniques including embossing into the polymer substrate. In a
particularly preferred embodiment, in step (a), the focussing
element array is formed by: (a)(i) applying at least one
transparent curable material to a focussing element support layer
(which is preferably transparent) or to a casting tool carrying a
surface relief corresponding to the focussing elements, at least
over an area corresponding to that of the first region; (a)(ii)
forming the transparent curable material(s) with the casting tool;
and (a)(iii) curing the transparent curable material(s) so as to
retain the surface relief; wherein either the focussing element
support layer comprises the polymer substrate or step (a) further
comprises applying the focussing element support layer to the first
surface of the polymer substrate, at least across the first region.
This process is frequently referred to as cast-curing.
Advantageously, the at least one transparent curable material is
applied to the focussing element support layer or to the casting
tool only over the area corresponding to that of the first region
and the casting tool carries the surface relief over an area
extending beyond that of the first region, preferably over
substantially the whole area of the casting tool. In this way the
lateral size and shape of the focussing element array can be
determined solely by the application of the curable material, with
the surface relief being formed by a standard casting tool. This
enables differently shaped focussing element arrays to be formed
using the same equipment through control of the application process
only, making the method well adapted for the production of devices
which are customised, e.g. to a particular series of banknotes,
without having to produce a specific casting tool for the
purpose.
The required optical spacing between the focussing element array
and the image array could be provided by another component of the
security document, e.g. the polymer substrate, but in preferred
embodiments, the surface relief on the casting tool is configured
such that the thickness of the formed transparent curable material,
optionally plus that of the focussing element support layer, is
substantially equal to the focal length of the formed focussing
elements, whereby preferably the focal plane of the array
substantially corresponds to the first surface or the second
surface of the polymer substrate. In other words, the surface
relief cast into the curable material includes an optical spacing
region between the focussing elements and the opposite surface of
the curable material. This may amount to the entire focal length,
in which case the curable material may itself provide all the
optical spacing necessary between the focussing elements and the
image array, or could be only part of it. In the former case this
enables the entire security device (i.e. the focussing elements and
the image array) to be provided on one side of the polymer
substrate. This is particularly desirable in dual-sided
configurations as discussed below but can also be beneficial e.g.
if it is not desired to include a window in the design of the
security document.
Preferably the casting tool comprises a cylinder carrying a sheet
in which the surface relief is defined on its circumference, the
casting tool cylinder being registered to the application of the at
least one opacifying layer such that the join between the ends of
the sheet on the cylinder falls within an area of the polymer
substrate that has been or will be covered with the at least one
opacifying layer. In this way any incomplete focussing elements or
interruptions in the focussing element array can be hidden and do
not detract from the appearance of the document.
In an alternative to cast-curing, in step (a) the focussing element
array may be formed by printing a doming resin on to a focussing
element support layer in accordance with the desired array pattern,
the doming resin having a surface energy configured such that the
doming resin adopts a profile capable of focussing light upon
deposition onto the focussing element support layer, wherein either
the focussing element support layer comprises the polymer substrate
or step (a) further comprises applying the focussing element
support layer to the first surface of the polymer substrate, at
least across the first region. Examples of suitable doming resins
can be found in U.S. Pat. No. 7,609,451 or US-A-2011/0116152, and
include UV curable polymer resins such as those based on
epoxyacrylates, polyether acrylates, polyester acrylates and
urethane acrylates. Examples include Nasdar.TM. 3527 supplied by
Nasdar Company and Rad-Cure.TM. VM4SP supplied by Rad-Cure
Corporation.
In another alternative, in step (a) the focussing element array may
be formed by treating the surface of a focussing element support
layer so as to vary its surface properties, preferably its surface
energy, in accordance with the desired array pattern and applying a
transparent substance over the focussing element support layer,
whereby the transparent substance reticulates due to the treated
surface so as to form the focussing elements. Further details as to
how the surface may be treated to vary the surface properties and
examples of suitable materials are given in US-A-20130071586.
In some preferred implementations, the focussing element array
includes focussing elements of different focal lengths, preferably
having different heights. This can be used to increase the
complexity of the optical effects achieved.
The method may advantageously further comprise applying a
camouflaging layer over the image array between the image array and
the viewer on the opposite side of the security document from that
on which the focussed image of the image array not visible, the
camouflaging layer preferably comprising a layer of iridescent,
colour-shifting or liquid crystal material, optionally a patterned
layer. This improves the appearance of the device from the side of
the document on which the focussed image is not visible.
In one particularly preferred embodiment, the focussing element
array and the image array are provided on the same side of the at
least one opacifying layer, the at least one opacifying layer lying
between the image array and the polymer substrate, and the
focussing element array and the image array extend across at least
25% of the whole area of the polymer substrate, more preferably at
least 50%, still preferably at least 70% and most preferably
substantially the whole area of the polymer substrate. This enables
the optical effect to be exhibit across a large area, rather than
being confined to a window region, thereby increasing the visual
impact. The document may still be provided with standard (static)
graphics, e.g. security fine-line patterns, portraits and the like,
which may be printed on the at least one opacifying layer or over
the focussing element array. If the former, the image array is
preferably configured to have a low optical density (e.g. fill
factor) so that it does not significantly obstruct the appearance
of the static graphics. For example a moire magnifier or integral
imaging type array of microimages is particularly suitable for use
as the image array in this case.
The security documents and security devices of the current
invention can be optionally be made machine readable by the
introduction of detectable materials in any of the layers or by the
introduction of separate machine-readable layers. Detectable
materials that react to an external stimulus include but are not
limited to fluorescent, phosphorescent, infrared absorbing,
thermochromic, photochromic, magnetic, electrochromic, conductive
and piezochromic materials. Preferably one or more of the at least
one opacifying layer comprises an electrically conductive material,
to provide anti-static properties. These optional features apply to
all aspects of the invention.
The invention further provides a security document made in
accordance with the above method, preferably a banknote, cheque,
identification document, passport, visa or stamp.
The following aspects of the invention are not limited to use in
the above described method, although this is preferable.
One aspect of the present invention provides a method of applying a
pattern to a pattern support layer, comprising the steps of: (b)(i)
providing a die form, the die form having a surface comprising a
plurality of recesses defining the pattern; (b)(ii) applying a
first curable material to the surface of the die form such that it
substantially fills the plurality of recesses and removing any
excess of the first curable material from outside the recesses on
the surface of the die form; (b)(iii) bringing a pattern support
layer in contact with the surface of the die form such that it
covers the plurality of recesses; and (b)(iv) separating the
pattern support layer from the surface of the die form such that
the first curable material in the plurality of recesses is removed
from the plurality of recesses and retained on the pattern support
layer in the form of a plurality of printed features which
correspond to the pattern; wherein each of the plurality of printed
features projects away from the pattern support layer to form a
raised surface of the plurality of printed features, and wherein
the plurality of printed features are separated from each other by
gaps in the first curable material on the pattern support layer;
the method further comprising the steps of (b)(v) during and/or
after step (b)(ii), at least partly curing the first curable
material in one or more curing steps; and (b)(vi) applying at least
one optically detectable material to at least a portion of the
pattern support layer either such that the at least one optically
detectable material is present on only the raised surface of the
printed features, and is substantially not present in the gaps in
the first curable material on the pattern support layer, which
separate the printed features, or such that the at least one
optically detectable material is present on only in the gaps in the
first curable material on the pattern support layer, which separate
the printed features and is substantially not present on the raised
surface of the printed features.
As mentioned in connection with the first aspect of the invention,
this method allows for the formation of a high resolution pattern,
e.g. with features of 10 microns or less, the appearance of which
is determined by the at least one optically detectable material.
This method can be utilised to form such patterns for any purpose,
including image arrays as mentioned above, but also for security
features which do not involve focussing elements, such as microtext
features or moire interference devices. The method can also be used
in the context of making devices (such as moire magnifiers etc) in
the context of a security article such as a thread, strip, patch or
the like, which can then be applied to or incorporated into a
security document of any type, e.g. a paper-based document.
In a particularly preferred embodiment, step (b) further comprises,
after step (b)(ii) and before step (b)(iii): (b)(ii') covering the
surface of the die form and the recessed areas filled with the
first curable material with a second curable material; wherein step
(b)(v) further comprises at least partly curing the second curable
compound, and in step (b)(iii) the pattern support layer contacts
the second curable material on the surface of the die form such
that in step (b)(iv) the second curable material is additionally
retained on the pattern support layer, the first curable material
being retained on the second curable material in accordance with
the pattern. This technique has been found to improve the bonding
between the pattern elements and the pattern support layer.
Alternatively, step (b) may advantageously further comprise, before
step (b)(iii): (b)(ii'') applying a second curable material to the
pattern support layer; wherein step (b)(v) further comprises at
least partly curing the second curable compound, and in step
(b)(iii) the second curable material on the surface of the pattern
support layer contacts the die form such that in step (b)(iv) the
first curable material is retained on the second curable material
in accordance with the pattern. In this case it is desirable that
the pattern support layer carrying the second curable material is
pressed against the surface of the die form with some pressure to
ensure good bonding.
In some embodiments, the second curable material will be
transparent so that the pattern can be viewed from either side
(provided the pattern support is also transparent). However in some
preferred implementations, the second curable material is a
substantially non-transparent material in the visible spectrum, the
non-transparent second curable material preferably forming one of
the at least one opacifying layers applied in step (c). Thus, the
second curable material may extend over substantially the whole of
the polymer substrate except for any desired window regions. The
said opacifying layer can be used as an optical barrier in a
dual-sided device as described in a later aspect of the
invention.
For the same reasons mentioned previously, preferably in step
(b)(vi) at least two optically detectable materials with different
optical detection characteristics are applied to the at least a
portion of the pattern support layer, preferably in laterally
offset sub-portions. Advantageously, wherein the at least two
optically detectable materials are applied to the portion of the
pattern support layer in register with one another. However this
register need not be highly accurate but only within the extent
which would be visible to the naked eye. In some preferred
implementations, the at least two optically detectable materials
are each applied sequentially to a transfer surface and then
applied together from the transfer surface to the portion of the
pattern support layer, the transfer surface preferably comprising
an offset roller or transfer blanket. Alternatively the materials
could be applied sequentially to the pattern, e.g. in multiple
print workings. Preferably the different optical detection
characteristics are any of: different visible colours, different
fluorescence, different luminescence or different phosphorescence.
Most advantageously a multi-coloured pattern or image of the
materials is applied.
Another aspect of the present invention provides a method of making
a security device, comprising: (a) forming an array of focussing
elements on a first region of a focussing element support layer,
which first region is less than the whole area of the focussing
element support layer, by: (a)(i) applying at least one transparent
curable material either to the focussing element support layer or
to a casting tool carrying a surface relief corresponding to the
focussing elements, over an area which includes the first region
and a second region laterally offset from the first region, the
area preferably encompassing the whole area of the focussing
element support layer or of the casting tool; (a)(ii) forming the
transparent curable material(s) with the casting tool; (a)(iii)
curing the transparent curable material(s) only in the first region
and not in the second region, so as to retain the surface relief in
the first region; and (a)(iv) removing the uncured transparent
curable material(s) from the second region.
This approach allows the lateral shape, size and location of the
focussing element array to be determined by the area of curing.
This can be selected through appropriate control of the curing step
and therefore allows the application of the curable material and
the forming of the focussing element to be carried out in a
standard manner, using standard tools, whilst achieving differently
shaped arrays. This lends itself well to customisation of the
device, e.g. for a particular series of banknotes, without needing
to manufacture a new casting tool, for example.
Advantageously, the transparent curable material(s) are curable by
exposure to radiation of at least a first wavelength, preferably UV
radiation, and step (a)(iii) is performed by exposing the
transparent curable material(s) to radiation of at least the first
wavelength through a patterned mask which defines the first region
as a radiation-transmissive portion thereof. The mask can be
formed, for instance, using well known demetalisation techniques or
laser ablation. Preferably, the surface relief and mask are
arranged on opposite sides of the focussing element support layer,
and are both configured to move at substantially the same speed as
one another and as the focussing element support layer as the
focussing element support layer is conveyed there past, the surface
relief and mask each preferably being carried on respective
opposing cylinders. This allows for continuous, web-based patterned
curing.
In other implementations, the radiation source can be a radiation
beam, e.g. laser, the direction of which can be controlled and then
scanned across the transparent curable materials in the first
region to achieve the desired patterning without a mask.
Preferably, the first region defines indicia, preferably
alphanumeric character(s), symbol(s), logo(s), graphics or the
like. This can be used to increase the complexity of the
device.
Another aspect of the present invention provides a method of making
a security device, comprising: (a) forming an array of focussing
elements on at least a first region of a focussing element support
layer, by: (a)(i) applying at least one transparent curable
material either to the focussing element support layer or to a
casting tool carrying a surface relief corresponding to the
focussing elements, over an area which includes at least the first
region; (a)(ii) forming the transparent curable material(s) with
the casting tool; and (a)(iii) curing the transparent curable
material(s) so as to retain the surface relief at least in the
first region; wherein the casting tool comprises a belt carrying
the surface relief, the belt being configured to move at
substantially the same speed as the focussing element support layer
along at least a part of a transport path along which the focussing
element support layer is conveyed, which part includes a section in
which the focussing element support layer is between an upstream
cylinder and a downstream cylinder, said section of the transport
path preferably being relatively planar, and step (a)(iii) is
performed while the belt and the focussing element support layer
traverse said section of the transport path.
By providing the surface relief on a belt as opposed to on a
cylinder (as in conventional cast-cure techniques), curing can take
place along a portion of the transport path which is relatively
open and hence allows for the provision of a greater number of
curing units than can generally be located in the vicinity of a
cylinder (where access to the focussing element support layer is
necessarily restricted). Hence curing can be performed with a
higher intensity of radiation and therefore more quickly and/or
more completely. Preferably step (a)(iii) is performed using at
least two curing energy sources, preferably radiation sources,
spaced from one another along the section of the transport path
and/or on both sides of the transport path.
In some preferred implementations the belt is formed as an endless
loop supported around at least two rollers, the belt being
separated from the focussing element support layer after step
(a)(iii) and retained for forming of transparent curable material
on a subsequent portion of the focussing element support layer.
In a particularly preferred embodiment, the belt is formed as a
transfer component which remains on the focussing element support
layer after step (a)(iii) and may optionally be removed in a
separate process. That is, the transfer component is not
necessarily reused but can be discarded after removal.
Another aspect of the present invention provides a method of making
a security device, comprising: (a) forming an array of focussing
elements on a first region of a focussing element support layer,
which first region is less than the whole area of the focussing
element support layer, by: (a)(i) applying at least one transparent
curable material either to the focussing element support layer or
to a casting tool carrying a surface relief corresponding to the
focussing elements, over an area which includes the first region
and a second region laterally offset from the first region, the
area preferably encompassing the whole area of the focussing
element support layer or of the casting tool; (a)(ii) forming the
transparent curable material(s) with the casting tool; and (a)(iii)
curing the transparent curable material(s) in the first region and
in the second region, so as to retain the surface relief in the
first region; wherein the surface relief is configured such that in
the cured transparent material(s) the highest parts of the
focussing elements in the first region are level with or below the
height of the cured transparent material(s) in the second
region.
In this way the focussing element array is provided alongside (and
preferably surrounded by) another portion of the curable material
which has a height greater than or equal to the maximum height of
the focussing elements. That is the focussing elements are
ultimately level with or depressed beneath the surface of the
adjacent material. This not only acts to protect the focussing
elements to a degree but also provides a substantially level
surface onto which a at least one opacifying layer can be applied
if the focussing element support layer is the polymer substrate of
a security document. This improves the application of that
opacifying layer since techniques such as gravure printing can then
be utilised without problems that would otherwise be caused by a
substrate of a varying thickness passing through the gravure
nip.
In some preferred implementations the surface relief is configured
such that in the second region the surface of the cured transparent
material(s) is substantially planar. This is particularly desirable
where an opacifying layer is to be applied thereover. However, in
other cases this could result in a high shine surface which is not
desirable, especially if the curable material is applied over any
opacifying layer or otherwise ultimately comprises the outermost
layer of the document or device. Hence, advantageously in the
surface relief is configured such that in the second region the
surface of the cured transparent material(s) carries a light
diffusing matt structure. This scatters light so as to present a
matt surface. In this and other cases where the height of the
material in the second region may vary from point to point, the
height of the focussing elements is preferably equal to or less
than the greatest height of the material in the second region.
In this aspect of the invention it is particularly desirable that
the focussing elements are concave focussing elements, defined as
depressions in the surface of the cured transparent material.
This aspect of the invention further provides a security device,
comprising an array of focussing elements formed of at least one
curable transparent material disposed across a first region of a
focussing element support layer, wherein the at least one curable
transparent material additionally extends across a second region of
the focussing element support layer laterally offset from the first
region and the highest parts of the focussing elements in the first
region are level with or below the height of the curable
transparent material(s) in the second region.
Any of the methods disclosed above may advantageously further
comprise:
(b) providing an image array located in a plane spaced from the
array of focussing elements by a distance substantially equal to
the focal length of the focussing elements whereby the focussing
elements exhibit a substantially focussed image of the image
array.
Likewise, any of the security devices or security documents
disclosed above may advantageously further comprise an image array
located in a plane spaced from the array of focussing elements by a
distance substantially equal to the focal length of the focussing
elements whereby the focussing elements exhibit a substantially
focussed image of the image array.
The image array can be configured to co-operate with the focussing
element array to produce an optically variable effect, e.g. of any
of the types mentioned above.
Another aspect of the present invention provides a method of
manufacturing a security device, comprising: providing a
transparent support layer having first and second surfaces, in the
form of a web; conveying the web along a transport path in a
machine direction; and during the conveying, simultaneously: (a)
forming an array of focussing elements on the first surface of the
transparent support layer in at least a first region; and (b)
applying an image array to the second surface of the transparent
support layer in at least part of the first region; whereby the
array of focussing elements and the image array are registered to
one another at least in the machine direction.
By performing steps (a) and (b) simultaneously on the same part of
the web, the array of focussing elements and the image array will
automatically be registered to one another at least in the machine
direction. In particular, any distortion suffered by the web, e.g.
due to the elevated temperatures that may be required for forming
the array of focussing elements, will be the same at the point of
forming the array of focussing elements and of applying the image
array.
In a particularly preferred embodiment, in step (a), a focussing
element cylinder carrying a surface relief on its circumference
corresponding to the array of focussing elements is used to form
the array of focussing elements on the first surface of the
transparent support layer, and in step (b), an image cylinder is
used to apply the image array to the second surface of the
transparent support layer, steps (a) and (b) being performed
simultaneously at a nip formed between the focussing element
cylinder and the image cylinder, the transparent support layer
passing through the nip. Depending on the location of this nip,
there may be substantially no (or low) pressure applied between the
two cylinders so as to avoid damage to the focussing elements.
Preferably, the transport path is configured such that the
transparent support layer is held in contact with the focussing
element cylinder over a portion of its circumference between a
first contact point and a last contact point spaced from one
another by a non-zero distance, wherein the nip formed between the
focussing element cylinder and the image cylinder either is located
between the first and last contact points, closer along the
transport path to the last contact point than to the first contact
point, or forms the last contact point. By positioning the nip in
this way, the focussing elements will be relatively fixed (e.g.
cured) relative to their state adjacent the first contact point, so
that a greater pressure can be applied at the nip, which typically
achieves a better outcome of the image array application
process.
In a particularly preferred embodiment, step (a) comprises: (a)(i)
applying at least one transparent curable material either to the
transparent support layer or to a casting tool, preferably a
focussing element cylinder, carrying a surface relief corresponding
to the focussing elements, over an area which includes at least the
first region; (a)(ii) forming the transparent curable material(s)
with the casting tool; and (a)(iii) curing the transparent curable
material(s) so as to retain the surface relief in the first
region.
Preferably the focussing element cylinder constitutes the casting
tool and step (a)(iii) is performed while the transparent support
layer is held in contact with the focussing element cylinder over
the portion of its circumference such that the at least one
transparent curable material is at least partly cured, preferably
fully cured, at the location of the nip between the focussing
element cylinder and the image cylinder.
The following aspects of the invention provide security documents
which can preferably be manufactured using any of the methods
disclosed above, but are not limited to such methods of
manufacture:
An aspect of the present invention provides a security document,
comprising: a transparent polymer substrate having first and second
surfaces; at least one opacifying layer on at least a portion of
the first surface of the polymer substrate; a first image array
disposed on the at least one opacifying layer, the at least one
opacifying layer being between the first image array and the
polymer substrate; a first focussing element array disposed over
the first image array, the first image array being between the
first focussing element array and the at least one opacifying
layer, the first image array lying substantially in the focal plane
of the first focussing element array whereby a substantially
focussed image of the first image array is displayed by the first
focussing element array; a second image array disposed either on
the first surface of the polymer substrate between the at least one
opacifying layer and the polymer substrate, or on the second
surface of the polymer substrate; and a second focussing element
array disposed over the second image array on the second surface of
the substrate, the second image array lying substantially in the
focal plane of the second focussing element array whereby a
substantially focussed image of the second image array is displayed
by the second focussing element array; wherein the at least one
opacifying layer substantially conceals the first image array from
the second focussing element array and the second image array from
the first focussing element array.
By using the at least one opacifying layer as a barrier between the
two image arrays, different optical effects can be viewed from the
two sides of the security document without interference. The first
image array may preferably be formed using the method defined in
steps (b)(i) to (b)(v) above, including step (b)(ii') or (b)(ii''),
the second curable material forming the at least one opacifying
layer.
Preferably, the at least one opacifying layer covers substantially
the whole first surface of the polymer substrate, optionally
excluding one or more window regions which preferably are laterally
offset from the first and/or second focussing element arrays.
Advantageously, the security document further comprises at least
one opacifying layer on at least a portion of the second surface of
the polymer substrate, excluding a window region in which at least
part of the second focussing element array is located.
Another aspect of the present invention provides a security
document comprising a substrate and, in a first region of the
substrate, a security device, the security device comprising: (a)
an array of focussing elements on a transparent support layer, the
transparent support layer comprising either the substrate or a
layer disposed thereon; and (b) an image array located in a plane
spaced from the array of focussing elements by a distance
substantially equal to the focal length of the focussing elements
whereby the focussing elements exhibit a substantially focussed
image of the image array; wherein the security document further
comprises a graphics layer extending across at least a second
region of the substrate laterally offset from the first region, the
graphics layer being configured to exhibit a first pattern, and the
image array is configured to exhibit the same first pattern,
whereby in the first region the focussing elements exhibit a
substantially focussed image of the first pattern which appears to
move upon changing the viewing angle relative to the static version
of the first pattern exhibited by the graphics layer in the second
region.
This provides a particularly strong visual effect which presents a
significant challenge to counterfeiters.
Preferably, the second region in which the graphics layer is
located is immediately adjacent the first region, advantageously
with the two regions abutting one another. The second region may
preferably surround the first region or vice versa. In particularly
preferred implementations, the second region may include at least
25% of the area of the security document, more preferably at least
50%, still preferably at least 70% and most substantially the whole
area of the substrate outside the first region.
Preferably, the size of the first pattern on the image array and
the magnification factor of the focussing elements are configured
such that the substantially focussed image of the first pattern
appears substantially the same size as the static version of the
first pattern.
Advantageously, the image array comprises a microimage array
constituting the first pattern, and the pitches of the focussing
element array and of the microimage array and their relative
orientations are such that the focussing element array co-operates
with the microimage array to generate a magnified version of the
microimage array due to the moire effect.
Another aspect of the present invention provides a method of
manufacturing a security document, comprising: providing a polymer
substrate having first and second surfaces in the form of a web;
and in any order: (c) applying at least one opacifying layer to the
first and/or second surfaces of the polymer substrate in the form
of a web, the or each opacifying layer comprising a non-transparent
material; and optionally either: (a) applying an array of focussing
elements to the first surface of the polymer substrate in the form
of a web across a first region; or (b) applying an image array to
the polymer substrate in the form of a web in the first region,
such that the image array is located in a plane spaced from the
array of focussing elements by a distance substantially equal to
the focal length of the focussing elements whereby the focussing
elements exhibit a substantially focussed image of the image array;
and then (q) cutting the web into sheets in the direction of web
transit, then performing on the sheets whichever of steps (a)
and/or (b) was not performed on the web, in at least one sheet-fed
process; such that either the image array is located between the
array of focussing elements and the at least one opacifying layer
on the first surface of the substrate, or at least the opacifying
layer(s) on the first surface of the substrate define a gap forming
a window region in which at least part of the array of focussing
elements is disposed such that a substantially focussed image of at
least part of the image array is displayed in the window
region.
By moving one or both of steps (a) or (b) to a point in the process
after the web has been cut into sheets, the efficiency of the
manufacturing process is enhanced. In particular, the techniques
and materials involved in steps (a) and (b) are typically
expensive, and may be slow, relative to other steps in the
manufacture. By carrying out one or both of these steps towards the
end of the process, and in particular after the web has been cut
into sheets, this not only increases the speed of the web-based
part of the process (since the rate at which this can be performed
is no longer limited by the speed of steps (a) and/or (b)), but
also reduces wastage. This is because steps (a) and/or (b) now need
only be performed on sheets which have met the required quality
criteria for all the preceding steps, such as application of the
opacifying layers (step (c)), rather than on the whole length of
the web, some of which might not reach the desired quality and
therefore might ultimately be discarded. By carrying out steps (a)
and/or (b) after "sheeting" as claimed, a greater number of the
processing steps will have been completed (and can be checked for
quality) before then performing steps (a) and/or (b). Ultimately
this reduces waste and saves time and costs.
Preferably both steps (a) and (b) are performed after the web has
been cut into sheets so as to maximise the above benefit. However,
in some cases, one of the steps (a) or (b) could be carried out
before step (q), i.e. as part of the web-based process, and only
the other is performed after sheeting. This still provides the
above benefit, although to a lesser degree.
In a particularly preferred implementation, the method further
comprises, after cutting the web into sheets and preferably before
performing on the sheets whichever of steps (a) and/or (b) was not
performed on the web: printing a graphics layer onto the at least
one opacifying layer on the first and/or second surfaces of the
polymer substrate in at least one sheet-fed process.
This is the conventional security print process which typically
involves applying security patterns (e.g. fine lines, guilloches),
portraits etc to a security document, e.g. by intaglio printing,
lithographic printing, flexographic printing or the like. It is
preferred that this step is completed before the sheet-fed
processes (a) or (b) so that these steps are moved still later in
the overall manufacturing process. This means that any sheets on
which the graphics layer is not printed to the necessary quality
can be removed before performance of steps (a) and/or (b), thereby
reducing wastage still further.
Preferably, in step (b) the image array is provided on the first
surface of the polymer substrate, the focussing element array
including an optical spacing portion. The optical spacing portion
can be formed integrally with the focussing element array by design
of a surface relief used to cast-cure the focussing element array
which option is discussed further above in connection with other
aspects of the invention.
Another aspect of the invention provides a method of manufacturing
a security document, comprising: providing a document substrate
having first and second surfaces in the form of a plurality of
sheets, each sheet optionally carrying one of: an array of
focussing elements on the first surface of the document substrate
or an image array; using a sheet-feeder to feed the plurality of
sheets one by one into a transport path; on the transport path,
performing either or both of the following steps in either order:
(a) applying an array of focussing elements to the first surface of
the document substrate in the form of a sheet across a first
region; and/or (b) applying an image array to the polymer substrate
in the form of a sheet in the first region, such that the image
array is located in a plane spaced from the array of focussing
elements by a distance substantially equal to the focal length of
the focussing elements whereby the focussing elements exhibit a
substantially focussed image of the image array.
This aspect of the invention is related to the previous aspect and
provides the same benefits. However, this aspect is not limited to
the use of polymer-based security documents, it being recognised
that the same advantages can be achieved where the method is
applied to any type of document substrate, e.g. polymer, paper or a
hybrid thereof. In this case the supplied substrate sheets may
already be provided with one or other of the focussing element
array or the image array (but not both), e.g. formed in a separate
web based process, optionally carried out by some different entity.
The other component (or both) is then provided in the sheet-fed
process as claimed by performing step (a) and/or (b).
Preferably, the method further comprises, after using a
sheet-feeder to feed the plurality of sheets one by one into a
transport path and preferably before performing on the sheets step
(a) and/or (b): printing a graphics layer onto the at least one
opacifying layer on the first and/or second surfaces of the
document substrate.
Again, it is preferred that the image array is provided on the
first surface of the polymer substrate, e.g. by means of the
focussing element array including an optical spacing portion.
Examples of security documents, security devices and methods of
manufacture thereof will now be described with reference to the
accompanying drawings, in which:
FIG. 1(a) shows an exemplary security document in plan view, FIGS.
1(b), (c) and (d) showing three alternative cross-sections along
the line X-X';
FIG. 2 is a flow diagram illustrating selected steps of a method of
manufacturing a security document according to one embodiment;
FIG. 3 schematically depicts exemplary apparatus for manufacturing
a security document in an embodiment;
FIGS. 4, 5, 6 and 7 show embodiments of apparatus for forming a
focussing element array, in each case illustrating (a) the
apparatus from a side view, and (b) a perspective view of the
focussing element support layer, FIG. 5(c) showing a further
variant of FIG. 5(a);
FIG. 8 illustrates an exemplary casting module that can be used in
any of the methods of FIGS. 4 to 8 and 11;
FIGS. 9(a) and (b) depict two embodiments of focussing element
arrays, showing (i) a surface relief suitable for the manufacture
thereof, and (ii) the resulting focussing element array disposed on
a support layer
FIG. 10 shows (a) an embodiment of a surface relief on a casting
tool suitable for use in any of the methods of FIGS. 4 to 8, (b) a
corresponding focussing element array formed on a support layer
using the surface relief, and (c) a focussing element array formed
on a support layer according to another variant;
FIGS. 11(a) and (b) show two examples of transfer components
comprising focussing element arrays which may be used in
embodiments of the invention, in cross-section;
FIGS. 12 to 15 schematically depict four embodiments of methods for
forming image arrays which may be used in embodiments of the
invention;
FIG. 16A to J shows examples of elements of image arrays formed as
relief structures;
FIG. 17 schematically depicts exemplary apparatus for manufacturing
a security document in an embodiment;
FIGS. 18(a) and (b) schematically depict selected components of
apparatus for manufacturing a security document in two further
embodiments;
FIGS. 19 and 20 depict two further embodiments of security
documents in cross-section; and
FIG. 21 depicts another embodiment of a security document (a) in
plan view and (b) in cross-section.
0. INTRODUCTION
The ensuing description will focus on preferred techniques for the
manufacture of security documents, such as bank notes, based on
polymer document substrates. However, many aspects of the
disclosure are more widely applicable and so should not be
considered limited to use on polymer-based security documents
unless otherwise indicated or necessitated by the nature of the
product or method in question. For example, many of the methods and
products described below can be utilised on security documents of
conventional construction, e.g. paper-based documents. For
instance, the described methods can be performed on a polymeric
support layer which can then be affixed to or incorporated into a
security document of any type. However, in all cases the preference
is for combination with a polymer-based security document.
0.1 Definitions
To aid understanding, the following terminology has been used
throughout the present disclosure: Polymer substrate--this refers
to a polymer document substrate which ultimately forms the main
body of a security document. Examples of such polymer substrates
are discussed in section 1 below. Focussing element array--this
refers to an array of elements capable of focussing visible light,
such as lenses or mirrors. The term "array of focussing elements"
is analogous. Examples are given in section 2 below. Image
array--this refers to a graphic which typically comprises a pattern
of microimages or image elements, although neither is essential. In
preferred cases the image array co-operates with a focussing
element array to generate an optically variable effect. For
example, the image array and the focussing element array may in
combination form a moire magnifier, an integral imaging device or a
lenticular device (each described above), or some other optically
variable device. In many preferred examples, the image array is
formed of elements of applied ink or another such material. However
this is not essential since the image array could instead be formed
of recesses or the like. Preferred methods of manufacturing image
arrays are discussed in section 3 below. Focussing element support
layer--this is a layer on the surface of which the focussing
elements are formed. The focussing element support layer could be
the polymer substrate (defined above) or could be another layer
which is then applied to a document substrate (paper or polymer),
or used as a carrier from which the focussing elements are later
transferred to a document substrate (paper or polymer). For
instance the focussing element support layer could take the form of
a security article such as a thread, strip, patch or foil which is
then incorporated into or onto a security document. Pattern support
layer--this is a layer on the surface of which the image array
(e.g. a pattern) is formed. The pattern support layer could be the
polymer substrate (defined above) or could be another layer which
is then applied to a document substrate (paper or polymer), or used
as a carrier from which the image array is later transferred to a
document substrate (paper or polymer). For instance the pattern
support layer could take the form of a security article such as a
thread, strip, patch or foil which is then incorporated into or
onto a security document. Transparent material--"transparent" is
used to mean that the material is substantially visually clear,
such that an item on one side of the material can be seen sharply
through the material from the other side. Therefore transparent
materials should have low optical scatter. However, transparent
materials may nonetheless be optically detectable (defined below),
e.g. carrying a coloured tint. Optically detectable
material/optical detection characteristics--an optically detectable
material may or may not be transparent but is detectable either to
the human eye or to a machine via an optical detector (e.g. a
camera), or both. Thus, the optical detection characteristic(s) of
the material could be for example a visible colour, a non-visible
reflection or absorbance such as UV or IR reflection or absorbance,
or a photoluminescent response such as fluorescence or
phosphorescence (the stimulating radiation and/or the emitted
radiation being visible or invisible), or the like. Curable
material--"curable" means that the material hardens (i.e. becomes
more viscous and preferably solid) in response to exposure to
curing energy which may for example comprise heat, radiation (e.g.
UV) or an electron beam. The hardening involves a chemical reaction
such as cross-linking rather than mere physical solidification,
e.g. as is experienced by most materials upon cooling.
0.2 Overview of Exemplary Security Document
For reference throughout the description of preferred manufacturing
processes below, FIG. 1 shows an exemplary security document 1,
such as a banknote, based on a polymer substrate construction. FIG.
1(a) shows the document in plan view and FIGS. 1(b), (c) and (d)
show three alternative cross-sections along the line X-X'. It will
be appreciated that the constructions shown are merely exemplary
and alternative arrangements are viable, some of which will be
discussed with reference to particular preferred manufacturing
techniques discussed below.
The security document 1 is based on a polymer substrate 2 which is
preferably transparent but this is not essential in all
embodiments. Examples of suitable polymer substrates 2 and optional
features thereof are described in Section 1 below. The polymer
substrate 2 has a first surface 2a and a second surface 2b. It
should be noted that wherever components are described herein as
being "on" one of the surfaces of the polymer substrate 2, or
actions are described as being performed "on" one of said surfaces,
this does not require the component or action to be directly on the
surface of the polymer substrate. Rather, some intermediate layer,
such as a primer layer, could exist immediately on the surface of
the polymer substrate itself and the component or action may be
applied to or performed on that intermediate layer, unless
otherwise specified.
On at least one of the surfaces of the polymer substrate 2,
preferably both, one or more opacifying layers 3a, 3b (indicated
generally as 3 in FIG. 1(a)) are provided. The opacifying layers
typically cover a large proportion of the surface area of the
security document 1, in some cases the entire area (as in FIG.
1(c), described below), but in other cases being omitted on one or
both sides of the polymer substrate 2 in localised areas to form
window regions. An exemplary window region 5 is shown in FIGS.
1(a), (b) and (c) but is omitted in the FIG. 1(d) variant. The
opacifying layer(s) 3 are configured to provide a suitable
background for a graphics layer 8, typically applied by printing,
which in the case of a banknote generally comprises secure fine
line patterns such as guilloches, a portrait, currency and
denomination information and the like. Thus the opacifying layers 3
are non-transparent and, in the case of a transparent polymer
substrate 2, act to increase the opacity of the document 1 as a
whole.
If the opacifying layers 3 are omitted in the window region 5 on
both sides of the polymer substrate 2, as shown in FIG. 1(b), the
window region will be a "full window" and, provided the polymer
substrate is transparent, will itself be transparent. If the
opacifying layers are omitted in the window region 5 on one side of
the polymer substrate 2 but not the other, the window region will
be a "half window" which is non-transparent but typically of lower
opacity than the surrounding regions of the document 1. An example
of a half window is shown in FIG. 1(c) in which the first
opacifying layer(s) 3a on the first surface 2a of the polymer
substrate 2 are absent in the window region 5 but the second
opacifying layer(s) 3b on the second surface 2b are continuous
across the window region 5. It will be appreciated that the window
region 5 could contain a mixture of full and half window areas by
arranging the gaps in the first and second opacifying layers to
overlap one another only partially (not shown). In FIG. 1(c) there
is no window, both opacifying layers 3a and 3b being continuous
across region 5.
Examples of suitable materials for forming the opacifying layer(s)
3 and more detail as to preferred methods for their application are
discussed in Section 4 below.
The security document 1 is provided with a security device 10 which
comprises at least an array of focussing elements 20 provided on
the first surface of the polymer substrate 2. The security device
10 could consist exclusively of the focussing element array 20 or
may also comprise an image array 30 as discussed below. In the
constructions of FIGS. 1(b) and 1(c), the focussing element array
is applied in a gap defined by the first opacifying layer 3a such
that the security device 10 is located in a window region 5 as
discussed above. However this is not essential and FIG. 1(d) shows
an example where the focussing element array 20 is applied to the
first surface 2a of the polymer substrate 2 over the first
opacifying layer(s) 3a. Preferred methods for manufacturing the
focussing element array 20 are discussed in Section 2 below, as
well as preferred configurations of the focussing element array 20
itself.
The image array 30, if provided, is preferably located in a plane
which substantially corresponds to the focal plane of the focussing
elements array 20 (e.g. to within +/-10%, more preferably +/-5%) so
that the focussing element array 20 exhibits a substantially
focussed image of the image array 30, which is illustrated
schematically by the broken-line sun-shaped outline in FIG. 1(a).
In practice this focussed image may be optically variable, i.e.
have different appearances at different viewing angles, and as such
may be referred to more generally as the "optical effect" exhibited
by the security device 10. For instance, the image array 30 could
co-operate with the focussing element array 20 to form a moire
magnification device, an integral imaging device or a lenticular
device, the principles of each having been discussed above, or any
combination thereof. Preferred methods of manufacturing the image
array 30, as well as examples of its configuration, are discussed
below in Section 3.
The focussing element array 20 and image array 30 can be provided
at various different positions provided the necessary spacing
between them is achieved. In the FIG. 1(b) example, this spacing is
provided at least in part by the polymer substrate 2 itself, which
here is transparent. The focussing element array 20 is located on
the first surface 2a of the polymer substrate 2 whilst the image
array 30 is located on the second surface 2b. It will be
appreciated that whilst FIG. 1(b) shows the device 10 as being
located in a full window, the second opacifying layer(s) 3b could
continue across all or part of the window region 5 (over the image
array 30), forming a half window or a mixture of full and half
window portions.
In the FIG. 1(c) example, both the focussing element array 20 and
the image array 30 are provided on the first surface 2a of the
polymer substrate 2, which now need not be transparent (although
this is still preferred). The optical spacing is provided by means
other than the polymer substrate 2 and exemplary methods for
achieving this are discussed in Section 2 below. In this case the
focussing element array 20 and image array 30 are located in a gap
in the first opacifying layer(s) 3a which forms a half-window.
However, the second opacifying layer(s) 3b could also be at least
partially omitted across the window region 5 to form a full window
or a mixture of full and half window portions.
In the FIG. 1(d) example, the focussing element array 20 and image
array 30 are again both provided on the first surface 2a of the
polymer substrate 2, this time over the first opacifying layer 3a
since as previously indicated no window is formed in this case.
Again the optical spacing is achieved by means other than use of
the polymer substrate 2 as will be discussed in section 2 below. It
will be appreciated from the FIG. 1(d) example, in which the
polymer substrate need not be transparent, that whilst security
devices 10 of the sort disclosed herein are particularly well
suited to application to documents based on polymer substrates,
they are not limited in this regard and can be utilised on any type
of security document, e.g. those based on paper substrates, or
indeed on any article which requires protection from
counterfeiting.
Depending on the type of optical effect desired to be displayed by
the security device 10, accurate registration between the focussing
element array 20 and the image array 30 may or may not be
necessary. However this is highly desirable in certain cases and
preferred techniques for achieving registration will be discussed
in Section 5 below.
Optional additional features and some preferred examples of the
security device 10 will be discussed in Section 6 below. The
security documents and security devices disclosed herein can be
optionally be made machine readable by the introduction of
detectable materials in any of the layers or by the introduction of
separate machine-readable layers. Detectable materials that react
to an external stimulus include but are not limited to fluorescent,
phosphorescent, infrared absorbing, thermochromic, photochromic,
magnetic, electrochromic, conductive and piezochromic materials.
This applies to all embodiments of the invention.
Typically to form the finished security document 1, a number of
additional processes will take place, including printing of the
graphics layer 8 already mentioned above, as well as application of
any further security articles such as security threads, strips,
patches, foils or the like which may carry features such as
diffractive elements (e.g. holograms or Kinegrams), iridescent
material, colour-shifting material etc. One example of such an
applied security article is shown in FIG. 1 as strip 9. The
so-formed material (generally in the form of a web or a sheet, at
this stage, as discussed further below) will then be cut into
individual security documents 1. All of these process steps are
considered optional in the present disclosure and can be
implemented by conventional means as discussed briefly in section
0.3 below.
Finally, the various components of the security document 1
described above can be applied in different orders. Section 7
describes preferred orders of the steps involved in manufacturing
the security document.
0.3 Overview of Exemplary Manufacturing Method
Turning now to the manufacturing process, FIG. 2 is a flow diagram
illustrating, at a high level, the main process steps in an
exemplary implementation. It must be emphasised that the order of
the steps can be varied significantly, different benefits being
achieved depending on the sequence of steps adopted, as will be
discussed in Section 7. Therefore, FIG. 2 serves merely to
introduce the key steps involved in manufacturing a polymer-based
security document and should not be considered to limit the order
of those steps, except where otherwise indicated. It should further
be noted that all steps shown in dashed lines are optional.
Thus in step S101, a polymer substrate 2 is provided, typically in
web form. The polymer substrate 2 and optional treatment steps that
may be performed before any of the steps described below are
carried out, are discussed in Section 1.
In step S200, a focussing element array 20 is applied to the
polymer substrate on its first surface. This will be described in
Section 2 but for the time being it is sufficient to note that the
step S200 could involve actual formation of the focussing element
array, either on the polymer substrate or on an intermediate
component such as a security thread, strip or patch (indicated as
step S200a) which is then affixed to the polymer substrate. However
this is not essential since the focussing element array could be
formed in some separate process, possibly by a different entity, as
an article such as a security thread, strip or patch, in which case
the present step S200 need only involve affixing the pre-formed
focussing element array 20 to the polymer substrate 2. For this
reason, in the main part Section 2 describes preferred methods of
forming the focussing element array as taking place on a focussing
element support layer, which could be the polymer substrate 2 but
alternatively could be a carrier layer in such a component.
In step S300, an image array 30 is applied to the polymer substrate
as will be described further in Section 3. However, as in the case
of the focussing element array 20, similarly step S300 may or may
not involve the actual formation of the image array 30. That is,
step S300 may comprise forming the image array 30 either on a
surface of the polymer substrate or on an intermediate component
such as a security thread, strip or patch (indicated as step S300a)
which is then affixed to the polymer substrate. Alternatively the
image array could be formed in some separate process, possibly by a
different entity, as an article such as a security thread, strip or
patch, in which case the present step S300 need only involve
affixing the pre-formed image array 30 to the polymer substrate 2.
For this reason, in the main part Section 3 describes preferred
methods of forming the image array as taking place on a pattern
support layer, which could be the polymer substrate 2 but
alternatively could be a carrier layer in such a component.
Indeed, where the focussing element array 20 and the image array 30
are both formed away from the polymer substrate 2 and then applied
thereto, the focussing element array 20 and the image array 30
could each be formed as part of one and the same security article
(such as a thread, strip or patch) which can then be affixed to the
polymer substrate 2 in a single step. Thus the focussing element
support layer and the pattern support layer could be provided by a
single support layer. It is noted as an aside that security
articles equipped with a focussing element array 20 and an image
array 30 can be applied to any type of security document, not
necessarily one based on a polymer substrate.
Registration between steps S200 and S300 is described in section 5
below.
In step S400, the at least one opacifying layer(s) are applied to
the first and/or second surfaces of the polymer substrate 2. In
practice this may optionally take place in several steps, which
need not all be performed immediately sequentially, one after the
other. For instance, one or more of the opacifying layers could be
applied before steps S200 and/or S300. Application of the
opacifying layer(s) is discussed in section 4 below.
In step S500, which is optional, the graphics layer 8 is applied to
the opacifying layers, typically by way of security printing
techniques. For example, the graphics layer 8 may be printed by any
conventional printing technique, or combination of techniques, such
as intaglio printing, lithographic printing, offset printing,
flexographic printing, gravure printing and the like. The graphics
layer 8 typically comprises high resolution patterns such as fine
line patterns and guilloches, portraits, and other indicia. In step
S600, which is also optional, any additional security devices on
articles such as threads, strips, patches etc., are applied to the
substrate. Any conventional techniques for applying such components
can be utilised, including bonding by adhesives, lamination, hot
stamping, transfer methods and the like. The security devices could
be of any known type, such as holograms, kinegrams and other
diffractive elements, iridescent or colour-shift material, etc.
Steps S500 and S600 could take place in either order and/or as a
series of sub-steps which could be intermingled with one another.
Finally, the processed material is cut into individual security
documents in step S700.
In the present example, all of the steps described have been
web-based processes, i.e. applied to a web of the polymer substrate
2, e.g. in one in-line process. Typically a web with a large width
(e.g. between 0.75 and 1.5 m) is supplied for this purpose.
However, for some process steps it is desirable to reduce the width
of the web, e.g. so that shorter (and hence less costly) processing
tools can be used. It is also desirable to carry out certain
process steps on individual sheets of the material, rather than on
a continuous web. This is particularly the case for security print
step S500. Hence, line S800 represents slitting the initial web
along its longitudinal direction so as to reduce its width,
subsequent processing steps utilising processing tools of
correspondingly shorter width compared with those of the preceding
steps. Line S900 represents dividing the web into sheets, by
cutting it along its cross direction at intervals spaced in the
longitudinal direction. This process is sometimes referred to as
"sheeting". Each sheet will preferably be sized so as carry a
plurality of the final security documents. Subsequent processes are
performed using sheet-fed machinery.
It will be appreciated that the points in the process at which
steps S800 and S900 are performed can be varied and are indicated
only schematically in FIG. 2. Typically at least one process step
will be performed on the reduced-width web (i.e. between steps S800
and S900), although this is not depicted here. Preferences will be
discussed in section 7.
In each of sections 1 to 8 below, several different options for
implementing each of the process steps will be described. It will
be appreciated that any one of the disclosed options in each
section can be combined with any one of the options disclosed in
each of the other sections. For example, any of the options
disclosed for forming the focussing element array 20 in section 2
can be used in combination with any of the options disclosed for
forming the image array 30 in section 3.
0.4 Overview of Exemplary Manufacturing Apparatus
To illustrate the production of various key components of the
security document 1 by the above steps, FIG. 3 schematically
illustrates exemplary apparatus for carrying out steps S200, S300
and S400 on a polymer substrate 2 in the form of a web. It will be
noted that the order of steps shown here is different from that in
FIG. 2. The polymer substrate 2 is provided from a supply such as a
reel 100. As described in section 1 below the polymer substrate may
undergo various treatment steps (not shown in FIG. 3) before being
subjected to the processing described below. The polymer substrate
is conveyed along a transport path by a transport module (not
shown) of conventional construction. The direction of transit is
termed the machine direction (MD) and the orthogonal direction in
the plane of the web is the cross direction (CD).
At a focussing element station 200, a focussing element array 20 is
applied to the first surface of the substrate. As mentioned above,
this may involve actual forming of the focussing element array 20
in-situ on the polymer substrate, e.g. by cast-curing, or could
involve supplying a security article 290, shown in the form of a
thread or strip, from an ancillary supply 200a and affixing at
least portions of it carrying a pre-formed focussing element array
to the surface of the polymer substrate, e.g. by lamination,
adhesive or hot-stamping. Further details of preferred methods for
forming the focussing element array 20 are described in section 2
below. In the example shown, the focussing element array 20 is
applied at spaced intervals so as to form one or more devices 10 on
each section of the web that will form a separate security document
once cut. However in other cases the focussing element array could
be applied continuously along the polymer substrate 2.
At an opacifying layer station 400, one or more opacifying layer(s)
are applied to the first and/or second surfaces of the polymer
substrate 2, as described further in section 4 below. Since the
focussing element array 20 has already been applied to the polymer
substrate in this embodiment, the application of the first
opacifying layer 3a should omit at least part of the area(s) in
which the focussing element array 20 is disposed so that it remains
at least partially uncovered. The exception is where the focussing
element array comprises mirrors rather than lenses in which case it
could be covered on the first surface of the substrate and
ultimately viewed from the opposite side. In the example shown, the
second opacifying layer 3b is also omitted in the same area, so as
to form a full window in which the focussing element array 20 is
arranged.
At an image array station 300, an image array 30 is applied to the
second surface of the polymer substrate 2. As mentioned above, this
may involve actual forming of the image array 30 in-situ on the
polymer substrate, e.g. by printing, or could involve supplying a
security article 390, shown in the form of a thread or strip, from
an ancillary supply 300a and affixing at least portions of it
carrying a pre-formed image array to the surface of the polymer
substrate, e.g. by lamination, adhesive or hot-stamping. Further
details of preferred methods for forming the image array 30 are
described in section 3 below. In the example shown, the image array
30 is applied opposite each of the focussing element arrays 20 such
that in each window the device 10 exhibits a focussed image of the
image array 30.
The web can then go on to be subjected to any of the optional
processing steps described previously with respect to FIG. 2, not
shown in FIG. 3. As noted above, whilst the apparatus shown in FIG.
3 is depicted as an in-line, web-based process, it is not essential
that all of steps S200, S300 and S400 be carried out in such a way
as described below in section 7.
1. Polymer Substrate
The polymer substrate 2 forms the structural basis of the finished
security document 1 and is typically provided initially in the form
of a quasi-continuous web, e.g. of width between 0.75 and 1.5 m and
typically many tens or hundreds of metres long. The thickness of
the polymer substrate is preferably in the range 50 to 100 microns,
preferably 60 to 80 microns and most preferably about 70
microns.
In most embodiments, the polymer substrate 2 is transparent
although this is not essential in all cases. The polymer substrate
2 comprises one or more polymeric materials, typically
thermoplastics, such as: polypropylene (PP) (most preferably
bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET),
polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC),
nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin
Copolymer (COC), or any combination thereof. The polymer substrate
2 may be monolithic, e.g. formed from a single one of the above
materials, or multi-layered, e.g. having multiple layers of the
same type of polymer (optionally with different orientations) or
layers of different polymer types.
As mentioned previously, by "transparent" it is meant that the
polymer substrate is substantially visually clear, although it may
carry a coloured tint and/or another optically detectable substance
such as a fluorescent material.
One or both surfaces of the polymer substrate 2 may be treated to
improve adhesion/retention of subsequently applied materials. For
example, a primer layer may be applied to all or part of either
surface of the polymer substrate 2, e.g. by printing or coating.
The primer layer is preferably also transparent and again could be
tinted or carry another optically detectable material. Suitable
primer layers include compositions comprising polyethylene imine,
hydroxyl terminated polymers, hydroxyl terminated polyester based
co-polymers, cross-linked or uncross-lined hydroxylated acrylates,
polyurethanes and UV curing anionic or cationic acrylates.
Alternatively or in addition to the application of a primer layer,
the surface of the polymer substrate 2 may be prepared for onward
processing by controlling its surface energy. Suitable techniques
for this purpose include plasma or corona treatment.
The application of the primer layer(s) and/or other surface
treatment steps may be carried out as part of the processing steps
described below in sections 2 to 4, e.g. before the application of
material to the substrate 2, potentially in line with those
processes. Alternatively, the application of the primer layer(s)
and/or other surface treatment steps could be carried out
separately such that the pre-treated polymer substrate 2 is
supplied to the security document manufacturing process ready for
the application of material thereto.
2. Application of Focussing Element Array
A focussing element array 20 comprises a plurality of focussing
elements, typically lenses or mirrors, arranged over an area
typically in a regular one-dimensional or two-dimensional grid. The
nature of the focussing elements will depend on the desired optical
effect but examples include cylindrical focussing elements,
spherical focussing elements, aspherical focussing elements,
elliptical focussing elements, Fresnel focussing elements and the
like. The focussing elements can operate on refraction, diffraction
or reflection (in the case of mirrors). For brevity, in the
discussion below the term "lens" is used interchangeably with the
term "focussing element" but this should not be taken as
limiting.
The focal length of the lenses is directly related to their size
(radius) and the available optical spacing must be taken into
account when designing the lens array. Generally, the relationship
between focal length f and lens radius r is:
.varies..DELTA..times..times. ##EQU00001## where .DELTA.n is the
difference in refractive index across the interface defining the
lens surface. In an example, for an image array 30 on the second
surface of the polymer substrate 2 to be focussed by a focussing
element array on the first surface of the polymer substrate 2, the
optical geometry must be taken into account when selecting the
thickness of the polymer substrate 2 (and any other optical spacer
layer that may exist between the focussing element array 20 and the
image array 30) and the dimensions of the lenses. In preferred
examples the thickness is in the range 50 to 100 microns, hence the
focussing element array should have a focal length in the same
range. The periodicity and therefore maximum base diameter (or
width, in the case of elongate lenses) of the focusing elements is
preferably in the range 5 to 200 .mu.m, more preferably 10 to 100
.mu.m and even more preferably 10 to 70 .mu.m. In other examples,
the focussing element array 20 and image array 30 may both be
arranged on the same side of the polymer substrate in which case
the available optical spacing is likely to be smaller (e.g. 5 to 50
microns) and hence the dimensions of the lenses will need to be
correspondingly reduced. The f number for the lenticular focusing
elements is preferably in the range 0.1 to 16 and more preferably
0.5 to 4.
The focussing element array 20 could include focussing elements
with different optical properties from one another, e.g. different
focal length, in different sub-regions of the array, by appropriate
design of the elements' shape and size. For example, the focussing
element array could include lenses of different height from one
another giving rise to different focal lengths in each region. In
such cases, if a focussed image of an image array 30 is desired the
image array 30 may be located at just one of the focal lengths, or
two image arrays 30 could be provided, one at each focal
length.
Preferred methods for manufacturing the focussing element array 20
will first be discussed in section 2.1, followed by preferred
configurations of the focussing element array in section 2.2.
2.1 Methods of Manufacturing a Focussing Element Array
Preferred methods of manufacturing the focussing element array 20
include direct embossing into the surface of the polymer substrate
2, cast-curing, printing and surface-treatment controlled coating
methods. Apart from the first of these, each of these techniques
can either be performed on the first surface of the polymer
substrate 2 or could be performed on another (transparent) support
layer which is then affixed to the first surface of the polymer
substrate 2. As defined above, the term "focussing element support
layer" is intended to cover both of these options and is therefore
used below. In places this is shorted to "support layer" for
brevity.
In one embodiment, lenses may be printed onto a support layer using
techniques such as those discussed in U.S. Pat. No. 7,609,451 or
US-A-2011/0116152. A doming resin is applied to the support layer
using a printing technique such as flexographic, lithographic or
gravure printing in accordance with the desired grid arrangement.
The nature of the doming resin and the volume in which it is
applied is configured such that, upon application, the material
adopts a dome-shaped profile having light-focussing properties.
Examples of suitable doming resins are mentioned in the above-cited
documents and include UV curable polymer resins such as those based
on epoxyacrylates, polyether acrylates, polyester acrylates and
urethane acrylates. Examples include Nasdar.TM. 3527 supplied by
Nasdar Company and Rad-Cure.TM. VM4SP supplied by Rad-Cure
Corporation.
In another embodiment, lenses may be formed by controlling the
surface energy of the support layer in accordance with the pattern
of lenses to be formed, and then applying a suitable material which
will reticulate in accordance with the varying surface energy to
form the lenses. Examples of how to implement this, and of suitable
materials, can be found in US-A-20130071568.
In a further embodiment, a surface relief defining the focussing
element array can be embossed into the surface of the polymer
substrate 2 from a suitably shaped embossing die, by the
application of heat and pressure. This approach has the advantage
that no additional layers of material need be applied to the
polymer substrate 2, thereby keeping its thickness to a minimum.
However, in some cases this is not beneficial since this reduces
the available optical spacing and hence requires the formation of
smaller lenses and (as discussed in section 3) higher resolution of
the image array 30.
The most preferred method of forming the focussing element array 20
is by cast-curing. This involves applying a transparent curable
material either to the support layer or to a casting tool carrying
a surface relief defining the desired focussing element array,
forming the material using the casting tool and curing the material
to fix the relief structure into the surface of the material. FIGS.
4 and 5 schematically depict two preferred cast-curing techniques
which may be used. Components common to both methods are labelled
with the same reference numbers. In both cases the process is shown
as applied to a focussing element support layer 201, comprising a
transparent film, which may be the aforementioned polymer substrate
2 or could be another layer which is later applied to the polymer
substrate 2. In each case, Figure (a) depicts the apparatus from a
side view, and Figure (b) shows the support layer in a perspective
view, the manufacturing apparatus itself being removed for clarity.
FIG. 5(c) shows a variant of the FIG. 5(a) embodiment.
In the FIG. 4 embodiment, a transparent curable material 205 is
first applied to the support layer 201 using an application module
210 which here comprises a patterned print cylinder 211 which is
supplied with the curable material from a doctor chamber 213 via an
intermediate roller 212. For example, the components shown could
form part of a gravure printing system. Other printing techniques
such as lithographic, flexographic, screen printing or offset
printing could also be used. Print processes such as these are
preferred since the curable material 205 can then be laid down on
the support 201 only in first regions 202 thereof, the size, shape
and location of which can be selected by control of the print
process, e.g. through appropriate configuration of the pattern on
cylinder 211. However, in other cases, an all over coating method
could be used, e.g. if the focussing element array is to be formed
all over the support 201 or if the method variants described below
with respect to FIGS. 6 and 7 are utilised. The curable material
205 is applied to the support 201 in an uncured (or at least not
fully cured) state and therefore may be fluid or a formable
solid.
The support 201 is then conveyed to a casting module 220 which here
comprises a casting tool 221 in the form of a cylinder carrying a
surface relief 225 defining the shape of the focussing elements
which are to be cast into the curable material 205. As each region
202 of curable material 205 comes into contact with the cylinder
221, the curable material 205 fills a corresponding region of the
relief structure, forming the surface of the curable material into
the shape defined by the relief. The cylinder 221 could be
configured such that the relief structure 225 is only provided at
regions corresponding to shape and position of the first regions
202 of curable material 205. However this gives rise to the need
for accurate registration between the application module 210 and
the casting module 220 in order that the focussing elements are
accurately placed in each first region 202 of the curable material.
Therefore in a particularly preferred embodiment, the cylinder 221
carries the relief structure corresponding to the focussing
elements over an area larger than that of the first region 202,
preferably around its complete circumference and most preferably
over substantially its whole surface (although axial regions which
will not come into the vicinity of the curable material may be
excluded). In this way, each entire first region 202 of curable
material 205 is guaranteed to come into contact with the surface
relief structure 225 such that the focussing element array is
formed over the full extent of the material. As a result, the
shape, size and location of the focussing element array 20 is
determined solely by the application of the curable material by the
application module.
Having been formed into the correct surface relief structure, the
curable material 205 is cured by exposing it to appropriate curing
energy such as radiation R from a source 222. This preferably takes
place while the curable material is in contact with the surface
relief 225 although if the material is already sufficiently viscous
this could be performed after separation. In the example shown, the
material is irradiated through the support layer 201 although the
source 222 could alternatively be positioned above the support
layer 201, e.g. inside cylinder 221 if the cylinder is formed from
a suitable transparent material such as quartz.
FIG. 5 shows variants of the above process in which, rather than
apply the curable material 205 to the support layer 201, it is
applied instead to the surface of the casting cylinder 225. Again
this is preferably done in a patterned manner, using a print
cylinder 211 to transfer the curable material 205 only onto the
first regions 202 on the casting cylinder 221. Upon contact with
the support layer 201, the regions 202 of curable material 205
affix to the support layer 205 and curing preferably takes place at
this stage to ensure strong bonding. The so-formed focussing
element arrays 20 again have a shape, size and location determined
solely by the application module 210.
FIG. 5(c) illustrates an alternative implementation in which rather
than apply the curable material 205 to the support layer 201 or the
casting cylinder 221 in a patterned manner to define the first
regions 202, the casting cylinder 221' is modified to achieve such
patterning. Thus, the surface relief 225 defining the focussing
element array is only provided in discrete patches of the surface
of the casting cylinder 221' with the intervening areas having no
surface relief. The curable material 205 can be applied all over
the surface of casting cylinder 221', e.g. from a reservoir as
shown or from an applicator roller. The curable material 205 fills
at least the relief regions 225 and if any is collected on the
intervening surface regions, a removal device such as a doctor
blade or squeegee 213' may be provided to clear those areas. The
support layer 201 is brought into contact with the cylinder 221',
preferably in a wrap configuration as shown, and the curable
material 205 is exposed to appropriate curing energy R from a
source 222, preferably during contact as shown. The support layer
201 is then separated from the cylinder 221' and now carries
discrete patches of focussing element arrays 20 in respective first
regions 202.
In all of the above embodiments, preferably the first regions 202
have the form of indicia, such as an alphanumeric character, a
symbol, logo or other item of information to increase the
complexity of the design.
The surface relief 225 may be carried by cylinder 221 in the form
of a sheet embossed or otherwise provided with the required relief,
which is wrapped around the cylinder 221 and clamped in place. This
may result in a noticeable join 225a where the two ends of the
sheet meet, at which there is a discrepancy in the relief pattern.
If replicated into one of the focussing element arrays this would
cause a reduction in quality. It is therefore preferred that the
casting module is at least coarsely registered to the application
module so that the location of join 225a where it contacts support
201 does not coincide with any of the first regions 202 but rather
is located between them, as shown by the example location labelled
225b. In cases where the curable material is applied (and retained)
all over the support, or at least along a continuous strip in the
machine direction MD, this join 225a is still preferably positioned
outside the first region which is to be used to form the security
device, advantageously in a location which will subsequently be
coated with one of the opacifying layers 3. To achieve this
consistently it is desirable for the process for forming the
focussing element array to be registered with the opacifying layer
application process, discussed in section 4, e.g. performed in the
same in-line process.
FIGS. 6 and 7 show an alternative cast-cure process for forming the
focussing element array. Again, components corresponding to those
described above are labelled with the same reference numerals used
previously and will not be described in detail again. In this case,
the shape, size and location of each focussing element array is
determined not by the initial application of the curable material
205 to the support layer 201 but by selective curing of that
material.
Referring first to FIG. 6, here the application module 210 applies
the curable material over not only the first regions 202 in which
the focussing element array is ultimately to be located, but
additionally over a second region 203 such that in this example
substantially the whole of the first surface of the support layer
201 is coated with the curable material 205. Thus whilst in the
example shown the application module is still constituted by a
printing system as described previously (but in which the cylinder
211 defines a print area substantially over the whole area of the
support as described here), this could be replace by a
non-selective, all over coating module. The curable material 205 is
then brought into contact with the casting tool 220 which again in
this case is preferably provided with the appropriate surface
relief 225 over substantially the whole of its circumference. Thus,
the whole of the first and second regions 202, 203 of the curable
material are formed in accordance with the relief structure.
However, only selected portions of the material are cured. This can
be achieved by providing a mask 223 through which the curable
material 205 is exposed to the curing energy, e.g. UV radiation.
The mask 223 defines radiation-transparent portions corresponding
to the first regions 202 and radiation-opaque portions in between
such that the second region 203 of the curable material is not
cured. In this example, the radiation source 222 is located inside
the casting cylinder 221 and the mask 223 is also arranged on the
inside of that cylinder.
A removal module 230 is additionally provided to remove the uncured
material 205 from the second region 203, leaving only the cured
material in the first regions 202, bearing the desired surface
relief and thereby forming the focussing element arrays 20. The
removal module 230 can comprise a cleaning roller 231 with a
(preferably soft) surface to which the uncured material 205 will
adhere and be lifted off the support 201. A cleaning system such as
a doctor blade or squeegee 232 may be provided to remove the waste
material 205 from the roller 231.
In a variant of the FIG. 6 embodiment, the patterned mask 223 and
curing energy source 222 may be arranged on the other side of the
transport path, as shown in FIG. 7. Here the support layer 201 is
conveyed through a nip defined between the casting cylinder 221 and
a mask cylinder 223 arranged to move at substantially the same
speed as one another. In other respects the FIG. 7 apparatus is the
same as that of FIG. 6.
In both variants, any join 225a in the surface relief on the
casting cylinder is preferably aligned with one of the opaque
portions of the mask 223 such that the area of material 205 into
which that part of the surface relief is formed will not be cured
and is removed by station 230.
In both variants, the curable material 205 could be applied to the
surface of the casting cylinder 221 instead of onto the support
later 201, e.g. using an arrangement corresponding to that shown in
FIG. 5.
In all of the above methods, the casting tool comprises a cylinder
221 carrying a surface relief 225 on its circumference. This is
convenient in many circumstances and has been found to achieve good
results. However, as already mentioned it is usual for such a
cylinder to exhibit a join 225a in its surface which, if steps are
not taken to avoid it, can result in some low quality focussing
element arrays being produced. Also, due to the space occupied by
the cylinder (and any opposing cylinder, not shown) there is a
limit on the size and number of curing units (e.g. radiation
sources) that can be provided to cure the curable material 205
while it is still in full contact with the surface relief on the
cylinder.
FIG. 8 shows an alternative implementation of the casting module
220 which can be used in any of the above embodiments or those
below. Here, the surface relief 225 is carried on a flexible belt
224, rather than a cylinder as in previous embodiments. The belt
224 is supported between at least two rollers 221a, 221b which
bring it into and then out of contact with the support layer 201 at
respective points P.sub.1, P.sub.2 spaced along the transport path
by a non-zero distance. The surface relief 225 is brought into
contact with the support layer 201 by the first roller 221a at
point P.sub.1, forming the regions of curable material 205 applied
thereto in accordance with the desired relief structure defining
the lenses. The surface relief 225 remains in contact with the
support layer 201 as both come off the roller and are conveyed
together towards point P.sub.2. During this section of the
transport path, the curable material 205 is cured, e.g. by
radiation R. The belt 224 is preferably transparent to the
radiation so that curing can take place from either or both sides.
Due to the increased space in which the curable material is held in
contact with the surface relief, a greater number and/or size of
curing units, e.g. sources of curing energy can be arranged to
effect curing, meaning that a more complete cure can be achieved
more quickly. This ensures the relief structure is fixed before the
cured material is removed from the casting tool.
The belt 224 can either be implemented as an endless loop, or as a
sacrificial, single-use component. For example, in the former case
the ends of the belt marked I and II are ultimately joined (not
shown) and the belt traverses a continuous loop supported on the at
least two rollers. Thus after being removed from the support layer
201 at point P.sub.2 the belt 224 is circulated back to point
P.sub.1 where it is brought into contact with another portion of
the support layer. Alternatively, the belt could stay in contact
with the support layer after roller 221b indefinitely. At some
later point the belt could be stripped off the support layer,
leaving the formed lenses. In a still further variant, if the
surface relief 225 and belt 224 are both transparent, and the
surface relief 225 is formed of a material with a refractive index
sufficiently different from that of the curable material 205, the
belt 224 could remain in contact with the focussing element array
and form part of the final assembly in the security document 1.
In all of the above methods, the transparent curable material 205
in which the lenses are formed can be of various different
compositions. The curable material 205 is preferably
radiation-curable and may comprise a resin which may typically be
of one of two types, namely:
a) Free radical cure resins, which are typically unsaturated resins
or monomers, pre-polymers, oligomers etc. containing vinyl or
acrylate unsaturation for example and which cross-link through use
of a photo initiator activated by the radiation source employed
e.g. UV. b) Cationic cure resins, in which ring opening (e.g. epoxy
types) is effected using photo initiators or catalysts which
generate ionic entities under the radiation source employed e.g.
UV. The ring opening is followed by intermolecular
cross-linking.
The radiation used to effect curing will typically be UV radiation
but could comprise electron beam, visible, or even infra-red or
higher wavelength radiation, depending upon the material, its
absorbance and the process used. Examples of suitable curable
materials include UV curable acrylic based clear embossing
lacquers, or those based on other compounds such as
nitro-cellulose. A suitable UV curable lacquer is the product
UVF-203 from Kingfisher Ink Limited or photopolymer NOA61 available
from Norland Products. Inc, New Jersey.
The curable material 205 could itself also be elastomeric and
therefore of increased flexibility. An example of a suitable
elastomeric curable material is aliphatic urethane acrylate (with
suitable cross-linking additive such as polyaziridine).
2.1 Focussing Element Array Configurations
As already indicated, the focussing element array typically
comprises a regular grid of elements, such as lenses or mirrors,
which may be cylindrical, spherical, apsherical, Fresnel or of any
other type necessary to achieve the desired visual effect. The
focussing elements can be concave or convex. The array
configuration may be modified to include any of the following
features to provide additional benefits. Each of these structures
can be formed using any of the above described methods (including
embossing, printing etc), but the cast-curing embodiments described
above are used for illustration.
FIG. 9(a) depicts an embodiment of a focussing element array 20,
FIG. 9(a)(i) showing a surface relief 225 suitable for the
manufacture thereof, and FIG. 9(a)(ii) showing the resulting
focussing element array 20 disposed on a support layer 201. The
location of an optional image array 30 is indicated.
In this example, the surface relief 225 is configured to include a
base 24 of height h.sub.B between the lenses 20 and the opposite
surface of the curable material 205 in which the focussing element
array is formed, by depressing the surface relief corresponding to
the lenses deeper into the casting tool. The base 24 improves the
mechanical stability of the focussing element array 20 and its
adhesion to the support layer 201 since the surface area of
material 205 in contact with the support layer 201 is increased and
the individual lens surfaces do not directly reach the surface of
the material 205. In this example the integrity of the array is
further enhanced by arranging the base 24 to extend beyond the
periphery of the focussing element array 20 itself at regions 24'.
The height h.sub.B will need to be taken into account, as well as
the lens height h.sub.I itself (i.e. the sagittal height) when
deciding the optical spacing between the focussing element array 20
and the image array 30 in order to ensure the image array 30 lies
as the desired focal distance f from the lens apex. In preferred
embodiments the height h.sub.B may be 10 microns or less, for
example, preferably 5 microns or less, most preferably between 1
and 3 microns.
FIG. 9(b) depicts another embodiment of a focussing element array
20, FIG. 9(b)(i) showing a surface relief 225 suitable for the
manufacture thereof, and FIG. 9(b)(ii) showing the resulting
focussing element array 20 disposed on a support layer 201. The
location of an optional image array 30 is indicated.
In this example, the surface relief 225 is configured to include an
optical spacing region 29 of height h.sub.s between the lenses 20
and the opposite surface of the curable material 205 in which the
focussing element array is formed, by depressing the surface relief
corresponding to the lenses deeper into the casting tool. This
enables the focussing element array itself to provide all or part
of the necessary focal length f between the lenses and the image
array 30. This is particularly useful where both the focussing
element array and the image array are to be provided on the same
surface of the security document 1. In preferred embodiments the
height h.sub.s is approximately equal to the focal length of the
focussing element array 20, e.g. 5 to 200 .mu.m, more preferably 10
to 100 .mu.m and even more preferably 10 to 70 .mu.m.
FIG. 10 shows embodiments in which the transparent material 205 is
applied and retained on the support 201 not only in the first
region 202 in which the focussing element array 20 is to be
located, but also in an adjacent second region 203 which may
optionally extend over the whole area of the support. The surface
relief 225 is configured such that in the second region 203 the
curable material 205 is retained with a height greater than or
equal to the maximum height of the focussing element array 20. In
the example shown in FIGS. 10(a) and (b) the surface relief 225 is
configured to provide gaps separating the first and second regions
of the material 205, but this is not essential. By arranging for
the surface of the lenses to be flush with or sit below the level
of the material 205 in the neighbouring region 203, the lenses are
relatively protected from damage during handling. Further the
generally flat resulting surface provides a good surface for later
printing of the opacifying layers 3 thereon, if desired.
FIG. 10(c) shows an alternative embodiment employing the same
principle in which the focussing elements are concave rather than
convex, as is preferred in this configuration.
2.3 Indirect Formation of Focussing Element Array
As noted in the introduction, the focussing element array 20 could
be formed directly on the polymer substrate 2 of the security
document 1, in which case the focussing element support layer 201
referred to in sections 2.1 and 2.2 will be the polymer substrate
2. Alternatively, the above-described methods could be performed on
another transparent carrier foil, forming the support layer 201, to
form a security article such as a thread, strip or patch. The
so-formed article can then be applied to a polymer substrate 2,
e.g. by lamination, adhesive or hot-stamping, to affix the
focussing element array 20 to the first surface of the substrate 2.
Alternatively, the article could be formed as a transfer element
from which the formed focussing element array 20 can be transferred
onto the substrate 2 and affixed thereto, leaving the support layer
201 behind, which can then be disposed of.
Two preferred constructions of transfer elements 290 are shown in
FIGS. 11(a) and (b). The focussing element array 20 is formed on a
transparent support layer 201 using any of the techniques described
above. The focussing element array 20 and support layer 201 are
then laminated to a carrier film 291 via a release layer 292, which
could comprise for example a pressure sensitive adhesive, a wax or
a primer layer. Preferably the release layer is thin, e.g. 0.2 to
0.3 microns so as to contact only a small area on the top of each
lens (or at the sides of each lens in a concave arrangement. An
adhesive layer 293 is provided on the opposite surface of the
support layer 201. Upon attachment to the polymer substrate 2, the
adhesive layer 293 is brought into contact with the first surface
2a of the substrate 2 and any additional steps required to achieve
bonding are carried out, e.g. heating and/or curing depending on
the nature of the adhesive. The carrier layer 291 and release layer
292 are then stripped off the focussing element array 20.
The variant shown in FIG. 11(b) is substantially the same as that
already described, except that here an overcoating 21 is provided
between the focussing element array 20 and the release layer 292,
so that the release layer 292 does not contact the lens surfaces.
The overcoating 21 could for example comprise a clear lacquer. The
overcoating will however need to have a different refractive index
from that of the material from which the focussing element array 20
is formed, in order to maintain the functionality of the lenses.
Preferably, the difference in refractive index is at least 0.1,
preferably at least 0.15. The overcoating 21 is retained on the
lenses when the carrier layer 291 and release layer 292 are
stripped off.
3. Application of Image Array
As noted in the introduction above, the provision of an image array
30 is optional but preferred. It is particularly advantageous to
provide an image array configured to co-operate with the focussing
element array 20 to produce an optically variable effect. For
example, the image array 30 and focussing element array 20 may, in
combination, form a moire magnification device, an integral imaging
device or a lenticular device, the mechanism on which each operates
having been discussed above.
Security devices of the above types depend for their optical effect
at least in part upon the high resolution with which the image
array 30 has been produced. For instance, in a lenticular device,
each image element or "slice" making up image array 30 must be
narrower than the pitch of the focussing element array 20, which as
discussed above is typically no more than 100 microns, usually
less. For example, if the diameter of the focusing elements is 30
.mu.m then each image element may be around 15 .mu.m wide or less.
Alternatively for a smooth lenticular animation effect it is
preferable to have as many different interleaved images as
possible, typically at least five but ideally as many as thirty. In
this case the size of the image elements should be in the range 0.1
to 6 .mu.m. In practice, in a lenticular device, the width of the
image elements is directly influenced by two factors, namely the
pitch of the focusing element (e.g. lens) array and the number of
image elements required within each lens pitch or lens base width.
The former however is also indirectly determined by the thickness
of the lenticular device. This is because the focal length for a
plano-convex lens array (assuming the convex part of the lens is
bounded by air and not a varnish) is approximated by the expression
r/(n-1), where r is the radius of curvature and n the refractive
index of the lens resin. Since the latter has a value typically
between 1.45 and 1.5 then we may say the lens focal approximates to
2r. Now for a close packed lens array, the base diameter of the
lens is only slightly smaller than the lens pitch, and since the
maximum value the base diameter can have is 2r, it then follows
that the maximum value for the lens pitch is close to the value 2r
which closely approximates to the lens focal length and therefore
the device thickness.
To give an example, for a security thread component as may be
incorporated into a banknote, the thickness of the lenticular
structure and therefore the lens focal length is desirably less
than 35 .mu.m. Let us suppose we target a thickness and hence a
focal length of 30 .mu.m. The maximum base diameter we can have is
from the previous discussion equal to 2r which closely approximates
to the lens focal length of 30 .mu.m. In this scenario the
f-number, which equals (focal length/lens base diameter), is very
close to 1. The lens pitch can be chosen to have a value only a few
.mu.m greater than the lens diameter--let us choose a value of 32
.mu.m for the lens pitch. It therefore follows for a two channel
one-dimensional lenticular device (i.e. two image element strips
per lens) we need to fit two image strips into 32 .mu.m and
therefore each strip is 16 .mu.m wide. Similarly for a four channel
one-dimensional lenticular the printed line width requirement drops
down to 8 .mu.m (in this example).
As a result, the f-number of the lens should preferably be
minimised, in order to maximise the lens base diameter for a given
structure thickness. For example suppose we choose a higher
f-number of 3, consequently the lens base diameter will be 30/3 or
10 .mu.m. Such a lens will be at the boundary of diffractive and
refractive physics--however, even if we still consider it to be
primarily a diffractive device then the we may assume a lens pitch
of say 12 .mu.m. Consider once again the case of a two channel
device, now we will need to print an image strip of only 6 .mu.m
and for a four channel device a strip width of only 3 .mu.m.
Similar considerations apply to other types of devices. For
example, in moire magnifiers and integral imaging devices, each
microimage must be of the same order of magnitude as one lens, or
smaller. Thus, the microimage will typically have overall
dimensions of 50 microns or less. In order to provide the
microimage with any detail, small line widths are required, e.g. of
15 microns or preferably less, ideally 5 microns or less.
Conventional printing techniques will generally not be adequate to
achieve such high resolution. For instance, typical printing
processes used to manufacture pattern elements (image arrays) for
security devices include intaglio, gravure, wet lithographic
printing and dry lithographic printing. The achievable resolution
is limited by several factors, including the viscosity, wettability
and chemistry of the ink, as well as the surface energy, unevenness
and wicking ability of the substrate, all of which lead to ink
spreading. With careful design and implementation, such techniques
can be used to print pattern elements with a line width of between
25 .mu.m and 50 .mu.m. For example, with gravure or wet
lithographic printing it is possible to achieve line widths down to
about 15 .mu.m. However, consistent results at this resolution are
difficult to achieve and in any case this level of resolution still
imposes a significant limitation on the security device. Thus while
any of the above-mentioned techniques can be employed in
embodiments of the present invention, higher resolution methods
(i.e. suitable for achieving smaller line widths) for forming the
image array 30 would be highly desirable.
Specialist high resolution printing techniques for forming image
arrays which can achieve smaller line widths are discussed below in
section 3.1
Another approach for forming high-resolution image arrays 30 is
through the use of relief structures, such as diffractive
structures, in place of ink-based processes. This approach can be
used in embodiments of the present invention and is discussed in
more detail below in section 3.2.
3.1 Print-Based Methods for Forming Image Arrays
One method which has been put forward as an alternative to the
printing techniques mentioned above, and can be employed in
embodiments of the invention, is used in the so-called Unison
Motion.TM. product by Nanoventions Holdings LLC, as mentioned for
example in WO-A-2005052650. This involves creating pattern elements
("icon elements") as recesses in a substrate surface before
spreading ink over the surface and then scraping off excess ink
with a doctor blade. The resulting inked recesses can be produced
with line widths of the order of 2 .mu.m to 3 .mu.m.
A different method of producing high-resolution image elements is
disclosed in WO-A-2015/044671 and is based on flexographic printing
techniques. A curable material is placed on raised portions of a
die form only, and brought into contact with a support layer
preferably over an extended distance. The material is cured either
whilst the die form and support layer remain in contact and/or
after separation. This process has been found to be capable of
achieving high resolution and is therefore advantageous for use in
forming the image array 30 in the present application.
Some more particularly preferred methods for generating patterns or
micropatterns (i.e. an image array 30) on a substrate are known
from US 2009/0297805 A1 and WO 2011/102800 A1. These disclose
methods of forming micropatterns in which a die form or matrix is
provided whose surface comprises a plurality of recesses. The
recesses are filled with a curable material, a treated substrate
layer is made to cover the recesses of the matrix, the material is
cured to fix it to the treated surface of the substrate layer, and
the material is removed from the recesses by separating the
substrate layer from the matrix.
Another strongly preferred method of forming a micropattern is
disclosed in WO 2014/070079 A1. Here it is taught that a matrix is
provided whose surface comprises a plurality of recesses, the
recesses are filled with a curable material, and a curable pickup
layer is made to cover the recesses of the matrix. The curable
pickup layer and the curable material are cured, fixing them
together, and the pickup later is separated from the matrix,
removing the material from the recesses. The pickup layer is, at
some point during or after this process, transferred onto a
substrate layer so that the pattern is provided on the substrate
layer.
The above-mentioned methods described in US 2009/0297805 A1, WO
2011/102800 and WO 2014/070079 A1 have been found to produce
particularly good results and are therefore particularly preferred
for use in forming the image array 30 in embodiments of the
invention.
FIG. 12 shows a preferred embodiment of a method for forming the
image array 30, which is based on the principles disclosed in WO
2014/070079 A1, where more details can be found. The image array is
formed on an image array support layer 301, which is preferably
transparent, and which could be the polymer substrate 2 ultimately
forming the basis of the security document 1, or could be another
carrier film which is then affixed to the security document 1. The
image array support layer 301 is preferably pre-primed, e.g. by
applying a primer layer such as a thin, optically clear UV adhesive
layer (not shown) or by raising its surface energy e.g. by corona
treatment. The desired pattern of image elements which are to form
the image array 30 (e.g. microimages, or slices of interleaved
images) is defined by recessed areas in the surface 303 of a die
form 302. Each recessed area preferably has a depth of the order of
1 to 10 microns, more typically 1 to 5 microns, and a width in the
range 0.5 to 5 microns. The recessed areas are separated by raised
areas of that surface 303. The die form preferably takes the form
of a cylinder, but this is not essential.
The recessed areas of the die form are filled with a curable
material 305, which is preferably visibly coloured (including
white, grey or black) but this is not essential and the material
could be colourless. The material 305 may or may not be
transparent. An exemplary first application module for applying the
material 305 into the recessed areas is shown at 310a. This
includes a slot die 312a configured to supply the curable material
305 to a transfer roller 311a from which it is applied to the die
form surface 303. The shore hardness of the transfer roller 311a is
preferably sufficiently low that some compression/compliance is
achieved to improve the transfer of material to the die form 302,
which is typically relatively rigid such as a metal print cylinder.
The applied ink layer should match or exceed the depth of the
recessed areas. The viscosity of the curable material may be
configured so that the material 305 transfers substantially only
into the recessed areas of the die form and not onto the raised
surfaces but in case any of the material 305 remains on the raised
surfaces it is preferred to provide a removal means such as doctor
blade 315a to remove any such excess material 305 from outside the
recessed areas. The material 305 in the recessed areas is
preferably then at least partially cured by exposing the material
305 to appropriate curing energy, e.g. radiation, from a source
320a, although this curing could be performed at a later stage of
the process.
Any suitable curable material 305 could be used, such as a
thermally-curable resin or lacquer. However, preferably, the
curable material is a radiation curable material, preferably a UV
curable material, and the curing energy source is a radiation
source, preferably a UV source. UV curable polymers employing free
radical or cationic UV polymerisation are suitable for use as the
UV curable material. Examples of free radical systems include
photo-crosslinkable acrylate-methacrylate or aromatic vinyl
oligomeric resins. Examples of cationic systems include
cycloaliphatic epoxides. Hybrid polymer systems can also be
employed combining both free radical and cationic UV
polymerization. Electron beam curable materials would also be
appropriate for use in the presently disclosed methods. Electron
beam formulations are similar to UV free radical systems but do not
require the presence of free radicals to initiate the curing
process. Instead the curing process is initiated by high energy
electrons.
Preferably the finished pattern is visible (optionally after
magnification) to the human eye and so advantageously the curable
material comprises at least one colourant which is visible under
illumination within the visible spectrum. For instance, the
material may carry a coloured tint or may be opaque. The colour
will be provided by one or more pigments or dyes as is known in the
art. Additionally or alternatively, the curable material may
comprise at least one substance which is not visible under
illumination within the visible spectrum and emits in the visible
spectrum under non-visible illumination, preferably UV or IR. In
preferred examples, the curable material comprises any of:
luminescent, phosphorescent, fluorescent, magnetic, thermochromic,
photochromic, iridescent, metallic, optically variable or
pearlescent pigments.
If the first application module 310a achieves substantially
complete filling of the recessed areas with material 305 then no
further application of curable material 305 may be required.
However it has been found that the recessed areas may not be fully
filled by a single application process and so, in particularly
preferred embodiments, a second application module 310b is provided
downstream of the first (and preferably of curing source 320a) for
applying more of the same material 305 to the die form. In the
example shown, the second application module 310b is of the same
configuration as the first, comprising a slot die 312b for
supplying the curable material 305 onto a transfer roller 311b
which applies the curable material 305 into the partially-filled
recessed areas on the die form surface. Again the viscosity of the
material could be adjusted so that it only fills those recessed
areas and is not substantially applied to the raised areas, but
preferably another removal means such as doctor blade 315b is
provided to remove any such excess material 305 from outside the
recessed areas. In the present embodiment, the transferred material
305 is then at least partially cured by second curing source 320b
although as discussed below this is not essential, or the degree of
curing of the additional material applied by second application
module 310b may be lower than that of the material applied
first.
If the recessed areas of the die form surface 303 are still not
substantially filled, third and subsequent application modules 310
can be provided as necessary.
Next, a tie coat 307 formed of a second curable material is applied
over substantially the whole surface of the die form 303, i.e.
coating both the filled recessed areas and the raised areas of the
surface 303. The second curable material may be of the same
composition as the first curable material but is preferably of a
different appearance (e.g. colour) so as to provide a visual
contrast with the first material in the finished array. In
particularly preferred embodiments, the tie coat composition may be
selected so as to improve the adhesion between the first curable
material and the support layer 301. The tie coat 307 is applied by
a tie coat application module 330 which here comprises a slot die
332 and a transfer roller 331. It is desirable for the tie coat to
be applied in a continuous, homogenous manner at the micron level
hence it is preferably applied in a metered way via a slot die and
transfer roller combination.
The tie coat may be partially cured at this point by a further
radiation source (not shown). The die form surface carrying the
filled recesses and tie coat is then brought into contact with the
support layer 301, either at a nip point or, more preferably, along
a partial wrap contact region between two rollers 309a, 309b as
shown. The combination is then exposed to curing energy, e.g. from
radiation source 335, preferably while the support layer 301 is in
contact with the die form surface. The support layer 301 is then
separated from the die form at roller 309b, carrying with it the
tie coat 307 and the elements of material 305 removed from the
recessed areas of the die form surface 303 by the tie coat 307. The
material 305 is therefore present on the support layer 301 in
accordance with the desired pattern, forming image array 30.
The tie coat 307 is preferably at least partially cured before the
die form 302 leaves contact with the support layer 301 at roller
309b, hence the preferred use of a partial wrap contact via lay on
and peel off rollers 309a, b as shown which tension the web around
the die form cylinder. If the material is not fully cured in this
step, an additional curing station may be provided downstream (not
shown) to complete the cure.
In a variant, after the tie coat 307 has been applied, a removal
means such as a further doctor blade could be provided to remove
the tie coat 307 from the raised portions of the die form surface
303 such that the regions of the tie coat 307 are confined to the
print images. These tie coat regions will most likely not be proud
of the die form surface. As such the support layer 301 in this
embodiment is preferably primed with a compliant adhesive layer
which may be partly cured prior to contacting the die form but
should still be compliant before entering the curing wrap.
Another embodiment of a method for forming an image array 30 is
shown in FIG. 13. In many respects this is the same as described
above with reference to FIG. 12 and so like items are labelled with
the same reference numbers and will not be described again. The
main difference is that here, the tie coat 307 is not applied to
the die form surface 303 but rather to the surface of support layer
301, upstream of the point at which it is brought into contact with
the die form. Thus the tie coat application module 330 is
positioned upstream and is configured to apply the material 307
over substantially the whole surface of support layer 301, either
directly, e.g. using a slot die 332 opposite an impression roller
333, or indirectly by applying the material 307 onto an offset
roller or transfer blanket (not shown) from which it is applied to
the support layer 301. The tie coat 307 could be applied to the
support layer by various other methods, including flexographic
printing or offset gravure, although these are less preferred since
they do not offer the same consistency and spatial homogeneity as a
slot die system.
The support layer 201 carrying the tie coat 307 is then brought
into contact with the die form surface so as to cover the filled
recessed areas and adjacent raised areas with the tie coat 307.
Preferably the tie coat 307 is pressed into the recessed areas so
as to achieve good joining therebetween before the curing process
begins. A second impression roller 334 may be provided for this
purpose, located after the lay on roller 309a but before curing
module 335.
In each of the methods described above, the recessed areas are
filled with curable material 205 in at least two application steps.
As described already, it is preferable to cure each application of
material 205 before the next is applied. The last application of
material may also be cured as described above. However, in a
further embodiment, additional benefits may be achieved by not
curing, or only partially curing, the last application of material
305 before it is brought into contact with the support layer 301.
In this way the last portion of material 305, located at the top of
each recessed area, remains relatively fluid and tacky at the point
at which it contacts either the tie coat 307 (if this is provided)
or the support layer 301. Once in contact, the material 305 can
then be fully cured by source 335. This has been found to result in
a particularly strong bond between the support layer 301 and the
pattern elements formed of material 305.
Whilst all of the above methods have been described with the use of
a tie coat 307, in fact this is optional but strongly preferred.
Hence the tie coat and its application steps may be omitted from
the above-described methods. This is particularly the case where
the last application of material 305 is not fully cured, as
described immediately above, since this incompletely cured material
can take on the function of the tie coat, helping to affix the
material 305 onto the support 301.
Where a tie coat 307 is provided, in particularly preferred
embodiments the material forming the tie coat 307 may contain an
anti-static additive, e.g. an electrically conductive substance.
This helps to disperse and therefore prevent the build-up of
electrostatic charge on the substrate, which in turn reduces the
tendency of the substrate to stick to other surfaces, including
other such substrates. Suitable anti-static materials for use in
the tie coat include graphite particles, as well as those
substances disclosed in EP1008616, WO2014/000020 and WO2008/042631.
In particularly preferred examples, the anti-static additive is
selected so as not to significantly modify the appearance of the
tie coat 307. Most advantageously, both the anti-static additive
and the tie-coat 307 as a whole may be visually transparent (i.e.
clear, but potentially carrying a coloured tint).
In many cases, the tie coat 307, if provided, will be a transparent
material such that the pattern formed by the material 305
transferred onto the support 301 can be viewed from either side.
However, this is not essential and in one advantageous embodiment,
the tie coat 307 could be non-transparent and configured to form
one of the opacifying layer 3 on the finished security document 1.
Thus, the tie coat 307 could comprise any of the opacifying
materials discussed below in section 3, if necessary with the
addition of a curing agent. The tie coat 307 could be applied in a
patterned manner, leaving gaps so as to form window regions if
desired. The patterned material 305 need not be applied all over
the tie coat but may be restricted to selected regions to form
localised image arrays 30, through appropriate configuration of the
surface relief on die form 302. The resulting image array 30 will
of course then be visible only from one side of the transparent
support layer 301 (which will be the polymer substrate 2 in this
scenario). However, this lends itself well to constructions of the
sort shown in FIG. 1(d) above, with the image array 30 and
focussing element array 20 both located on the same side of the
substrate 2. To provide the necessary optical spacing, the
focussing element array 20 could be formed using a surface relief
as described with reference to FIG. 9. Another example in which
forming the tie coat 307 as an opacifying layer 3 can be used to
beneficial effect will be described below with reference to FIG.
19.
Each of the above described methods will result in a pattern of
spaced elements of material 305 on the support layer 301,
optionally with an intermediate layer in the form of the tie coat
307 (if provided). Due to the manner in which the elements of
material 305 are formed, the pattern has a surface relief with the
elements of material 305 standing proud of the surface on which
they are arranged, with substantially none of the material 305
between them. The following embodiments of methods for forming an
image array 30 make use of this surface relief to modify the
appearance of the so-formed pattern.
FIG. 14 shows an extension of the method described above with
respect to FIG. 12 and those aspects already described in relation
to FIG. 12 will not be described again. It will be appreciated that
the presently described extension can be applied equally to the
output of the FIG. 13 method, or of any of the other variants
described above. Thus, the described method outputs a support layer
301 carrying a relief formed of the material 305 as mentioned. The
appearance of the pattern is now modified by applying one or more
optically detectable materials 345 to either the tops of the relief
structure (i.e. onto the tops of the elements of material 305), or
into the gaps between them. The same material must not be applied
to both, else the pattern will be lost. Hence, FIG. 14 shows a
station 340 which here is adapted to apply three different
optically detectable materials, e.g. three different colours of
ink, to only the tops of the material elements 305. In this case
the station 340 comprises three patterned print cylinders 346a,
346b and 346c each configured to apply a working of a different
material 341a, 341b, 341c so as to form a multi-coloured image. The
cylinders 346a, 346b, 346c are registered to one another in a
conventional way. Highly accurate register beyond that visible to
the human eye is not required since the high resolution patterning
is achieved by the upstream process of FIG. 12. The print cylinders
346a, b, c (each opposed here by impression rollers 347a, b, c) and
the materials 341a, b, c are configured in this example so as to
deposit the material only onto the tops of the elements 305 and not
into the gaps therebetween. This can be achieved for example by
controlling the viscosity of the materials 341a, b, c and/or
selecting a process such as flexographic printing in which the
material is applied to the print surface under only a light
pressure. In this way, the multi-coloured image is formed only on
the elements 305 and is absent elsewhere. The result is an image
array 30 comprising high-resolution pattern images which vary in
colour in accordance with the desired (macro) image, or any other
pattern. This can be used for instance to form a full colour
lenticular device which has previously proved extremely difficult
to manufacture.
The materials 341a, b, c may be curable materials in which case one
or more curing stations 348a,b,c may be positioned along the
transport path as necessary.
FIG. 15 shows a variant of the above process in which the optically
detectable materials 345a, 345b, 345c and 345d are applied to the
tops of the elements 305 in an indirect process. The materials are
applied in register with one another to a transfer roller 349 such
as an offset roller, to form the desired multi-coloured image
thereon. The materials are then applied to the elements 305 from
the transfer roller 349 in one step. This has been found to achieve
an increase resolution.
In both method variants, the optically detectable material(s) could
be placed only in the gaps between elements 305, rather than on
their tops. This can be achieved by changing the viscosity of the
materials and/or utilising a method in which the materials are
forced into the gaps and/or cleaned from the tops. In a still
further refinement, one optically detectable material could be
placed only in the gaps and a different optically detectable
material only onto the tops of the elements. More details as to how
the optically detectable materials may be applied, and suitable
types of materials, can be found in US20110045248.
3.2 Relief-Based Methods of Forming Image Arrays
In other examples the image array 30 can be formed by a relief
structure and a variety of different relief structure suitable for
this are shown in FIG. 16. Thus, FIG. 16a illustrates image regions
of the image elements (IM), in the form of embossed or recessed
regions while the non-embossed portions correspond to the
non-imaged regions of the elements (NI). FIG. 16b illustrates image
regions of the elements in the form of debossed lines or bumps.
In another approach, the relief structures can be in the form of
diffraction gratings (FIG. 16c) or moth eye/fine pitch gratings
(FIG. 16d). Where the image elements are formed by diffraction
gratings, then different image portions of an image (within one
image element or in different elements) can be formed by gratings
with different characteristics. The difference may be in the pitch
of the grating or rotation. This can be used to achieve a
multi-colour diffractive image which will also exhibit a lenticular
optical effect such as an animation through the mechanism described
above. For example, if the image elements had been created by
writing different diffraction tracks for each element, then as the
device is tilted, lenticular transition from one image to another
will occur as described above, during which the colour of the
images will progressively change due to the different diffraction
gratings. A preferred method for writing such a grating would be to
use electron beam writing techniques or dot matrix techniques.
Using a diffractive structure to provide the image elements
provides a major resolution advantage: although ink-based printing
is generally preferred for reflective contrast and light source
invariance, techniques such as modern e-beam lithography can be
used generate to originate diffractive image strips down to widths
of 1 .mu.m or less and such ultra-high resolution structures can be
efficiently replicated using UV cast cure techniques.
Such diffraction gratings for moth eye/fine pitch gratings can also
be located on recesses or bumps such as those of FIGS. 16a and b,
as shown in FIGS. 16e and f respectively.
FIG. 16g illustrates the use of a simple scattering structure
providing an achromatic effect.
Further, in some cases the recesses of FIG. 16a could be provided
with an ink or the debossed regions or bumps in FIG. 16b could be
provided with an ink. The latter is shown in FIG. 16h where ink
layers 200 are provided on bumps 210. Thus the image areas of each
image element could be created by forming appropriate raised
regions or bumps in a resin layer provided on a transparent
substrate. This could be achieved for example by cast curing or
embossing. A coloured ink is then transferred onto the raised
regions typically using a lithographic, flexographic or gravure
process. In some examples, some image elements could be printed
with one colour and other image elements could be printed with a
second colour. In this manner when the device is tilted to create
the lenticular animation effect described above, the images will
also be seen to change colour as the observer moves from one view
to another. In another example all of the image elements in one
region of the device could be provided in one colour and then all
in a different colour in another region of the device.
Finally, FIG. 16i illustrates the use of an Aztec structure.
Additionally, image and non-image areas could be defined by
combination of different element types, e.g. the image areas could
be formed from moth eye structures whilst the non-image areas could
be formed from gratings. Alternatively, the image and non-image
areas could even be formed by gratings of different pitch or
orientation.
Where the image elements are formed solely of grating or moth-eye
type structures, the relief depth will typically be in the range
0.05 microns to 0.5 microns. For structures such as those shown in
FIGS. 16a, b, e, f, h and i, the height or depth of the
bumps/recesses is preferably in the range 0.5 to 10 .mu.m and more
preferably in the range of 1 to 2 .mu.m. The typical width of the
bumps or recesses will be defined by the nature of the artwork but
will typically be less than 100 .mu.m, more preferably less than 50
.mu.m and even more preferably less than 25 .mu.m. The size of the
image elements and therefore the size of the bumps or recesses will
be dependent on factors including the type of optical effect
required, the size of the focusing elements and the desired device
thickness.
4. Application of Opacifying Layer(s)
Referring back to FIG. 1, the opacifying layer(s) 3 comprise a
non-transparent material, the primary purpose of which is to
provide a suitable background for later printing of graphics 8
thereon. Thus, preferably, the opacifying layers comprise
polymeric, non-fibrous material containing at least a light
scattering substance such as a pigment. The opacifying layers 3 are
preferably light in colour, most preferably white or another light
colour such as off-white or grey so that a later-applied graphics
layer 8 will contrast well against it. In preferred examples, the
opacifying layers each have a brightness L* in CIE L*a*b* colour
space of at least 70, preferably at least 80 and more preferably at
least 90. For example, each opacifying layer may comprise a resin
such as a polyurethane based resin, polyester based resin or an
epoxy based resin and an opacifying pigment such as titanium
dioxide (TiO2), silica, zinc oxide, tin oxide, clays or calcium
carbonate.
Two or more opacifying layers may be applied to each surface of the
polymer substrate 2, in order to achieve the necessary opacity. The
optical density of each layer by itself may typically be around 0.2
to 0.5. Preferably, 3 or more layers are applied to each surface,
overlapping one another.
In a preferred embodiment, at least one of the opacifying layers
(preferably one on each surface of the polymer substrate (2) is
made electrically conductive, e.g. by the addition of a conductive
pigment thereto. This reduces the effect of static charges which
may otherwise build up on the security document 1 during
handling.
The opacifying layers are preferably applied to the polymer
substrate using a printing process such as gravure printing,
although in other case the opacifying layers could be coated onto
the substrate, or applied by offset, flexographic, lithographic or
any other convenient method. Depending on the design of the
security document 1, the opacifying layers may be omitted across
gaps on one or both surfaces of the polymer substrate to form
window regions (which may be full windows or half windows, or a
mixture of both). This can be achieved through appropriate
patterning of the opacifying layers during the application
process.
In one preferred method, mentioned in section 2 above, an outer one
of the opacifying layers 3 can be applied as a tie coat 307 during
the application of an image array 30 thereon.
In alternative constructions, the opacifying layers 3 could
comprise self-supporting pre-formed layers (optionally including
apertures to later form windows) which are then laminated to the
polymer substrate 2. In this case, the opacifying layers could be
polymeric or could be of fibrous construction, such as paper, thus
rendering the security document a "hybrid" paper/polymer
construction.
5. Registration of Focussing Element Array and Image Array
In some cases, accurate registration of the focussing element array
20 and the image array 30 is not required, provided the two items
are at least coarsely registered to one another such that they
overlap in the desired device region. This is particularly the case
for moire magnification devices in which a magnified version of the
microimage array will be generated even if the two arrays are
misaligned, although the translational position and/or the
orientation and size of the magnified images may vary.
However, if registration can be achieved between the focussing
element array 20 and the image array 30, this enables a level
control over the optical effect generated by the device which is
extremely difficult to imitate by any other means and thereby
presents a substantial challenge to counterfeiters. For example, in
a moire magnification device, accurate registration enables the
precise location, size and orientation of the magnified images to
be maintained constant for every device manufactured such that a
user checking the authenticity of the device will be able to
compare the location of the magnified image to some reference point
on the security document (such as the centre of the device 10) and
if this is incorrect, reject the device as fraudulent.
Registration has even more profound effects on lenticular type
devices, in which the range of viewing angles over which each of
the interleaved images will be displayed depends on the positioning
of the respective image elements underneath each lens. It is
important to achieve good skew registration so that the orientation
of the two arrays are aligned. If not, parts of individual image
elements will extend from the footprint of the lens through which
they are intended to be viewed into another, with the result that
the desired images may not be displayed properly, or only across
part of the security device. In addition, without accurate
translational registration (in the machine direction and/or the
cross direction) of the focussing element array 20 to the image
array 30, it is not possible to control the location of the image
elements relative to the lenses meaning that the order in which
they will be displayed as the device is tilted cannot be
controlled. For instance, an image which is intended to be
displayed when the device is viewed along the normal may in
practice be displayed only at some off-axis angle, and images which
are intended to show different extremes of an animation (e.g. an
object at its largest size and at its smallest) may be displayed at
adjacent viewing angle ranges meaning that upon tilting the
animation appears to skip frames, jumping from one to another
without a smooth continuum therebetween. Various approaches for
avoiding this problem have been proposed, including the use of
cyclic effects as described in GB-A-2490780, in which the images
are configured such that the same cyclic animation will be
displayed no matter which image is located at the centre viewing
position. However, the lack of registration limits the type of
optical effect which can be implemented successfully. In
particular, sets of images showing the same object from different
view points so as to create a 3D effect upon tilting would benefit
greatly from accurate registration.
Some preferred methods for improving registration between the
focussing element array 20 and the image array 30 are discussed
below.
5.1 Mechanical Register
FIG. 17 shows an embodiment of the invention which provides good
skew register between the focussing element array 20 and the image
array 30, and offers an improvement in translational register
also.
The polymer substrate 2 is provided with at least one line of
apertures 51 spaced along the machine direction. Preferably two
such lines of apertures 51 are provided at either side of the web.
The manufacturing line may be provided with a die cutting module 50
for cutting the apertures 51 into the polymer substrate 2, e.g.
using a die cylinder 55 against an impression cylinder 56 with
corresponding recesses. Alternatively the polymer substrate 2 may
be supplied with the apertures 51 pre-cut.
In the focussing element array module 200, which is shown here
schematically but could take any of the forms discussed in section
2 above, either the casting tool 221 or an impression cylinder 57
provided to oppose it, is equipped with corresponding pegs 52
arranged to protrude along lines corresponding to the location of
the apertures 51 in the polymer substrate. In use, the pegs 52
extend through the apertures 51, holding the polymer substrate
square across its width as the focussing element array 20 is
formed.
The web 2 is then conveyed to an image array forming module 300,
which again is shown schematically but could take any of the forms
discussed in section 3 above. Either the die form 302 or an
impression cylinder 58 opposing it is provided with lines of pegs
52 which as before engage with the apertures 51 in the polymer
substrate as it is conveyed through the nip between the die form
and the impression cylinder, thereby holding the polymer substrate
square. As such, the degree of skew between the applied focussing
element array 20 and image array 30 is reduced.
It will be appreciated that the above technique involving the
engagement of the apertures and pegs can be employed no matter what
the order of the processing steps, and can also be utilised during
other steps such as application of the opacifying layers.
This method additionally achieves an improvement in the
translational registration of the components, but to a lesser
degree.
5.2 Simultaneous Application of Focussing Element Array and Image
Array
In preferred embodiments of the invention, the focussing element
array 20 and image array 30 are applied to opposite sides of a
transparent material, whether this be the polymer substrate 2 or
another support layer which can then be applied to the polymer
substrate 2 or to a conventional (e.g. paper-based) security
document, e.g. so as to form the structure shown in FIG. 1(d).
In such cases it is highly desirable for the focussing elements
array 20 and the image array 30 to be applied to the opposite
surfaces of the substrate simultaneously. That is, at the same
position along the transport path in the machine direction.
FIG. 18(a) shows an example of this in the case where the focussing
element array 20 and image array 30 are applied to the first and
second surfaces, respectively, of the polymer substrate 2. However
the same principles can be applied to the construction of an
article such as a security thread, in which case the substrate 2
will be replaced by some other, typically thinner, transparent
film. The focussing element array 20 and image array 30 can be
formed using any of the processes described above in sections 2 and
3. For clarity, FIG. 18(a) depicts only selected components of the
apparatus used to form the focussing element array 20 and image
array 30, namely a casting tool 221 (e.g. as shown in any of FIGS.
4 to 8) and a die form 302 (e.g. as shown in any of FIGS. 12 to
15). Other components of the process line are not shown. The
casting tool 221 and die form 302 are arranged on opposite sides of
the transport path along which the polymer substrate 2 is conveyed,
so as to form a (low pressure) nip through which the polymer
substrate 2 passes. At each location along the polymer substrate 2,
its first surface 2a therefore comes into contact with the casting
tool 221 at the same time as its second surface 2b comes into
contact with the die form 302. As a result, the focussing element
array 20 and image array 30 are applied to each point of the
substrate web simultaneously.
This has the significant advantage that any deformation experienced
by the polymer substrate 2, as a result of changes in processing
temperature or the like, will be exactly the same when the
focussing element array 20 is applied to the polymer substrate 2 as
it is when the image array 30 is applied. The web has no time to
expand or contract between the instant at which the focussing
element array 20 is applied and when the image array 30 is applied,
since they occur at the same time. As such, a high degree of
register between the two components is automatically achieved.
The arrangement shown in FIG. 18(a) has the disadvantage that since
the nip between the casting tool 221 and the die form 302
constitutes the first point of contact between the polymer
substrate and the casting tool 221, the transparent curable
material 205 from which the focussing element array 20 is formed
will be substantially uncured when it enters the nip. As such, the
pressure applied between the casting tool 221 and the die form 302
should be low so as to avoid damage to the cast focussing element
array 20.
FIG. 18(b) shows an improved arrangement in which formation of the
focussing element array 20 and application of the image array 30
can still be considered simultaneous because the curable material
205 is still in contact with the surface relief on casting tool 221
at the nip location between the casting tool 221 and the die form
302. The polymer substrate is wrapped around a portion of the
casting tool 221 from a first point at lay on roller 61, at which
casting of the focussing element array 20 begins, until the nip
with die form 302 at which point the focussing element array 20
will be relatively well cured, preferably fully cured. As such, the
pressure between the two components 221, 302 can be increased
relative to that in the FIG. 18(a) embodiment since the material
205 is relatively hard and less prone to damage. This improves the
quality achieved in the image array formation process. A further
benefit of the arrangement shown is the increased wrap length of
the substrate 2 around die form 302, allowing for prolonged curing
here also. The substrate 2 stays in contact with die form 302 from
the nip location until take-off roller 62.
6. Optional Additional Features and Preferred Examples
As mentioned above, whilst in many cases it is desirable to use the
polymer substrate 2 as the optical spacer between the focussing
element array 20 and the image array 30, this is not essential and
methods are provided above for arranging both components on one
side of the polymer substrate. This can be used to provide a number
of new effects.
A first example is shown in FIG. 19, in which opacifying layer 3a
on the first surface of polymer substrate 2 is used as an optical
barrier to separate two lens-based security devices, the first
formed by focussing element array 20a and image array 30a and the
second by focussing element array 20b and image array 30b. Image
array 30a is located on top of opacifying layer 3a, and focussing
element array 20 is positioned thereover. This could be
manufactured for example by using the methods described in section
3 above with a tie coat 207 formed of opacifying material to form
layer 3a. The focussing element array 20 could be formed with an
integral optical spacing portion to provide the necessary focal
length f.sub.a, as described in section 2 above. Alternatively, the
focussing element array 20 could be formed on a support layer 201
(not shown in FIG. 19) and the image array formed on the reverse
side of the support layer 201. Both can then be applied to the
opacifying layer by lamination or hot stamping.
Before the opacifying layer 3a is applied, however, the image array
30b must be formed on the first surface of polymer substrate 2 and
this could be achieved by any of the previously described methods.
Focussing element array 20b can also be formed on the second
surface of polymer substrate 2 using any of the methods described
in section 2. The polymer substrate 2 provides the necessary
optical spacing to achieve focal length f.sub.b.
FIG. 20 shows another example of a security document 1. Here, the
polymer substrate 2 is first provided with opacifying layers 3a
(and optionally 3b) and then printed with graphics layers 8a, 8b as
described above in section 0.3. Only then is security device 10
applied formed of image array 30 and focussing element array 20.
Each of these components can be formed using any of the methods
described above, e.g. lamination of a security sheet carrying both
components across the substrate 2. The image array 30 and focussing
element array 20 extend across substantially the whole surface, or
at least a significant part of, the security document 1 so as to
produce a strong visual impact. The image array 30 is preferably
configured to have a low fill factor so as not to significantly
obstruct the view of the underlying graphics layer 8a. For instance
the device 10 may be a moire magnifier or an integral imaging
device both of which are well suited to this application.
FIG. 20 also illustrates another optional feature which could be
provided in any embodiment of the invention, namely a camouflage
layer 90. This is provided to conceal the presence of an image
array 30 from the side of view from which the focussed version
thereof will not be visible. This is desirable since when viewed
without the focussing element array 20 the image array 30 is likely
to have a dull, indistinct appearance due to the very fine pattern
of microimages or image elements of which it is typically made up.
The camouflage layer can be formed of any suitable non-transparent
material, but metallic inks, iridescent inks or colour-shifting
inks are particularly preferred. The layer 90 may be uniform or
patterned, e.g. displaying indicia.
Another preferred example of a security document 1 is shown in FIG.
21, (a) in plan view and (b) in cross section. The construction of
the security document 1 is substantially the same as shown in FIGS.
1(a) and (b) although the security device 10 could alternative have
the construction shown in FIG. 1(c) or (d).
The image array 30 and focussing element array 20 forming the
security device 10 are configured so as to display a focussed image
(which may preferably be optically variable) across the region 5
(here a full window region) which is a pattern or a portion of
another image which is also displayed by the graphics layer 8
across a second region 6 of the security document 1 outside the
region 5. In the example shown the second region 6 encompasses the
whole area of the security document 1 outside window 5 but this is
not essential. Preferably however the region 6 abuts (i.e. borders)
the window region 5, and desirably surrounds it.
The two regions are configured so as to display the same image as
one another, preferably a repeating "wallpaper" type pattern,
although this is not essential and the window 5 could provide a
missing portion of any image the remainder of which is displayed in
region 6. However, the image in region 6 will appear static, since
it is formed by conventional means in graphics layer 8. In region 5
however, the image will be projected onto an image plane by the
focussing element array 20 and hence appears non-static, tending to
move relative to the image in graphics layer 8 upon tilting. This
effect can be enhanced further by forming device 10 as an optically
variable device such as a moire magnifier or a lenticular device,
in both cases projecting the same image as in region 6 but in a
manner which changes upon tilting the device. For example, a moire
magnifier exhibiting the image in the form of an array of magnified
microimages may give the impression of the array moving and the
images "scrolling" across the region 5 upon tilting. A lenticular
device can be arranged to animate the image, e.g. by moving it
around inside region 5, and/or by changing its colour.
Preferably the device 10 is configured such that the image is
displayed in region 5 at substantially the same size as in region
6, through control of any magnification factor applied by the
focussing element array 20 and the size of the elements forming
image array 30.
The result is a particularly distinctive security effect. It should
be appreciated that the above described image in region 6 will
typically not form the whole of graphics layer 8 but rather it will
be provided in addition to other features such as a portrait,
indicia etc. For instance, the image may provide a background to
such features.
7. Preferred Process Sequences
As noted at the outset, the various steps involved in manufacturing
the security document 1 can be carried out in different orders.
However, certain orders provide particular benefits and some
examples are given below. Also relevant is which steps are
performed on the initial web form in which the polymer substrate 2
are supplied, and which in a sheet-fed process after the web has
been cut into sheets.
Some preferred examples are now described, with reference to the
numbering of the process steps introduced in FIG. 2.
In a first preferred embodiment, key steps of the process are
carried out all in line, on a continuous web of the polymer
substrate 2. This has the benefit that the most accurate
registration between the various process can be achieved. Hence, in
one example first step S200 is performed to apply the focussing
element array 20 to the polymer substrate, using any of the methods
discussed in section 2 above. If any registration zones/marks are
to be used they should also be formed at this stage. If the
formation of the focussing element array 20 involves raised
temperatures, it is beneficial to perform this process first so
that any thermal distortion can be taken account of during the
later steps.
Next, the opacifying layer(s) are applied in step S400 (section 4),
e.g. by gravure, then the image array 30 is formed, e.g. using any
of the processes described in section 3. All of the above can be
performed on the initial web width, e.g. 0.75 to 1.5 m wide. If
necessary the web can then be slit to narrow it (step S800).
The web is then cut into sheets (step S900) and subsequently
printed and subject to any further finishing processes before
cutting into documents.
A second preferred option is substantially the same as above except
that step 200 is performed on a separate press (i.e. not in line)
before the opacifying layers and image array 30 are then applied to
the web (in line).
A third preferred option is substantially the same as the first
preferred embodiment above, but the image array 30 is applied after
the web has been narrowed (step S800), but before sheeting (step
S900). It is potentially more achievable to control web tension on
a narrower web (as opposed to the initial wide web) and therefore
higher registration and image resolution may be achieved in this
way.
In a fourth preferred option, either of the first or second options
above is modified by postponing formation of the image array 30
until after sheeting (step S900) and therefore typically in a print
works on a lithographic or flexographic press.
In a fifth preferred option, only the application of the opacifying
layers 3 (step S400) is performed on the web and the formation of
the focussing element array 20 and image array 30 are carried out
after sheeting (step S900), in a sheet-fed process. For example,
the image array 30 may be applied to the sheet first, on its first
surface, followed by a focussing element array 20 on the same
surface, e.g. incorporating an optical spacer. Any of the above
mentioned processes could be used. Preferably these steps take
place after printing of the graphics layer 8 and potentially after
any other security devices 9 have been applied.
By moving the focussing element array formation step and image
array formation step towards the end of the manufacturing process,
wastage and costs can be reduced. This is because both steps are
relatively slow and expensive compared to other steps of the
manufacturing process. By completing more of the other process
steps before the focussing element array 20 and image array 30 are
formed, these more costly steps need only be performed on sections
of the polymer substrate which have met the necessary quality
threshold in each of the preceding steps, and not on any waste
material.
Whilst it is preferred that both steps S200 and S300 are move
towards the end of the manufacturing process for this reason,
benefits are still achieved if just one or the other is postponed
in this way. Hence one of these steps could be performed on the web
(i.e. before sheeting S900) and only the other remaining one may be
performed on the sheets.
* * * * *